Author: Nicholas
Greetings, fellow Zerglings.
Over the past few days, there have a been a lot of topics asking why (insert Terran or Protoss strat here) keeps stomping Zerg. And among the responses, one thing is agreed upon - if you don't scout it out, you're toast. Always. So, that led to posts asking how to scout, when to scout, where to scout, such and such. However, I haven't seen any threads describing exactly what you might be seeing when you scout. So, I'm going to add that, and combine everything else into one big scouting post.
Scouting, without a doubt, is THE most crucial staple to Zerg play. It isn't Queen management, it isn't base macro, it isn't micro - it's scouting. If you don't scout, you will most likely lose. The reason behind this is that Zerg's units can be seen as highly specialized in that they excel at destroying anything they happen to be a counter to - however, they will be rolled over effortlessly if they come across a unit that counters them in the slightest. As such, if you know what units the enemy has, you're at an immediate advantage.
Here are the Zerg units that are, in my opinion, the best suited for scouting.
Zerglings - Zerglings are cheap, come in pairs, and they're fast as hell. However, they're extremely weak. Zerglings excel at scouting enemy expansions that aren't very well defended. However, any Protoss or Terran player worth his cheese will have a wall off or some kind of defense in front of his main base, so Zerglings are ineffective in that regard.
Changelings - These little guys are, in my opinion, the second most underused Zerg unit, behind Infestors. If you use them right, these guys are absolute gold. They're spawned from Overseers, and change shape into either a Marine, Zealot, or Zergling (depending on who your opponent is, of course) immediately upon enemy contact. I'll give some potential uses for these guys later on.
Drones - Drones are mainly used only as very early game scouts. Do not use them if you're anywhere past 20 supply, as they'll get killed before they can glean any useful information.
Roach - I was hesitant to put these on the list, as they're only efficient with burrow and tunneling claws. However, if you have both of those upgrades, you can burrow a roach and send him on towards the enemy army. If they have detection and he dies, it's not a huge deal, as they're cheap.
Overlord - These guys are often used by new players in all the wrong ways. I wouldn't call them scouts, even with the speed upgrade - I prefer to regard them as "observers". More about them later.
Now, here are some units that might seem good for scouting, but actually aren't.
Mutalisks - This is main reason I have this part of the guide. I see a lot of new players using Mutalisks to scout, as they're fast and can fly. However, they're extremely expensive, and can be put to much, much better uses harassing the enemy. Don't ever sacrifice these guys.
Banelings - This is self explanatory. Just use the Zergling you morphed it from and save yourself the gas.
Hydralisks - A lot of newer players may use these to scout because they are often the staple of a Zerg army, and they figure that the loss of only one is insignificant. Don't waste the gas when these can be much more efficiently used in an army.
Now that I've listed what units can scout, I'll describe exactly how they should go about doing so.
Zerglings - Like I said earlier, Zerglings are best used for scouting expansions. They won't make it into a main base before being killed. They are doubly effective at scouting expansions as if you can catch one early, it's easy to take it out.
Changelings - These are your main base scouters. However, like Zerglings, you can't send them through the front door. Next time you play a game, take a moment to look at your main base. There's probably a big cliff, or drop off that cannot be traversed, along the outside edge of the map. Naturally, you don't build anything there, right? Well, neither do Terran or Protoss players. So, take one of your Overlords (you should have plenty) and morph him to an Overseer. Let him hang out near the blind spot of your enemy's base, outside his field of vision. When you want to know what's up, move your Overseer right to the edge of the base, drop a Changeling, and get back out of sight. That Changeling will probably have free reign of the base, but if you're spotted and killed, you were probably close enough to his army to see what he was making anyways. Even if you only see one Banshee or one Colossus, remember that nobody just makes one of a unit and then forgets about them. If you see one of them, it's safe to assume there were more in the fog.
Drones - You will typically only send one Drone per game to scout. Depending on whether you build your Overlord at 9 or 10 supply, you'll send your 9th or 10th Drone to scout. (If 9 Ovie, send 10th Drone, if 10th Ovie, send 9th Drone. Duh.) Go ahead and send him right on in to your opponent's base, as he won't have anything that can kill it quite yet. Send him on a wild course of shift-clicking through the base. Make sure not to focus all of your attention on keeping him alive; keep your macro going, as you'll probably be expanding around this time. When the opponent builds a Marine or Zealot to attack your Drone, I like to send him to the nearest Vespene source and build an Extractor. Keep an eye on the Extractor - just as it finishes, cancel it, and you'll be targeting the Drone again by default. So, you cancel, then press "B" and "E" quickly, and build another. This is extremely annoying, and can often effectively neutralize a lot of annoying openers. Plus, less experienced players may panic and devote far too many units to killing your Extractor.
*Note that the information you gather with this Drone will likely be the most important information you glean for the entire game - later in the guide, I'll go over common Terran and Protoss build orders so you know what you're seeing.
Overlords - Possibly the most common mistake of new players is to send their initial Overlord directly to your opponents base. Do not do this. If you scout with your 9th or 10th Drone, he beats the Overlord there. By the time the Overlord gets there, a Terran player will have a Marine to shoot him down or a Protoss player will make a Stalker to shoot him down. Instead, send your first Overlord to your opponent's natural expansion. Send your second to the next closest expansion. Keep doing this until you've covered every mineral patch on the map. This is one of the most effective containment strategies you can use - undoubtedly, during midgame, you'll have a group of around 10 Zerglings that are just chilling, doing nothing. Keep them ready to strike - when one of your Overlords sees the enemy going for an expansion, move in. Target his SCV or Probe first, then take down the Nexus/Command Center. Your opponent just effectively wasted 450 minerals (if he didn't cancel - he probably will) and a good bit of time, and you still have an economic advantage (assuming you expanded early like most good Zerglings should). Another good idea is to keep an Overlord stationed far to the left of your opponent's base, and another far to the right. If you ever direly need to scout, send them both in at once, and you'll see something. Don't be afraid, in the late game, to send your Overlords on a suicide mission to scout, if that's your only option. They're cheap to replace, and if you have the speed upgrade (which you NEED TO HAVE) you'll likely get the information you need.
If any of you are wondering when you should be scouting, the answer is simple. Always. You don't have to be scouting every second of the game, but it should be one of the closest things to the front of your head as you play. Remember, it only takes about a minute and a half to build 6 or 7 units that you totally aren't prepared to face.
Now, for the final, and arguably most important, part of the guide. If you've been reading the forums, you should know that you always need to send a Drone at the beginning to scout. However, newer players that have only played Zerg likely won't even know what they're seeing when they see it. To finish up, I'm going to list some common Protoss and Terran openings that can be very annoying if not properly scouted.
Protoss
The Four-Gate - This is one of the most notoriously annoying Protoss openers, and absolutely deadly if you don't scout it and react quickly. In most games, your Drone will reach your opponents base when you have about 16 supply. At 16 supply, a Protoss going for a Four-Gate will probably have one Pylon, one Gateway, one Assimilator, and a Cybernetics Core on the way. This doesn't necessarily mean they're going for a Four-Gate, it could also be a Void Ray rush. As such, it's important to stick around with your one Drone for as long as possible when fighting a Protoss opponent. If he throws down a second Gateway, it's a Four-Gate. If it's a Four-Gate, he will also be repeatedly Chrono-Boosting his Cybernetics Core, researching the Warp Gate upgrade. When you see this happen, build a Roach Warren as soon as possible, and put up at least three Spine Crawlers at your natural. Build a healthy force of Zerglings with speed as well, and you should have no problem fending off this rush. Make sure you scout the area near your base and remove any proxy Pylons he may have laid down.
The Void Ray Rush - This opener is extremely deadly if you aren't prepared for it. If you don't see it coming, it will almost certainly lead to the GG. At 16 supply, a Void rush will actually be identical to the Four-Gate, probably - one Pylon, one Gateway, one Assimilator, and a Cybernetics Core. If he throws down a Stargate after the Cybernetics Core, it's Void Rays for sure - he might also throw down a second Assimilator. Even if you see this coming, it's sometimes a pain to deal with. Build a few extra Queens, and tech up to Hydralisks as quickly as possible. If you macro well, you should have 6-7 Hydralisks out by the time his first Void Rays even reach your base, and you should be good from there on out. Note that an early Stargate could also mean he is going for a Phoenix harass. This isn't as threatening to your base, however - if you don't kill Phoenixes, they will severely cripple your economy. Use the same counters that you would use against Void Rays - Queens are very effective against Phoenixes.
Fast Expand Builds - The Protoss typically won't go for these. If they do, however, you'll likely see a quick Forge, so they can throw down Photon Cannons immediately to protect their expansion. The best way to counter this is to take a third expansion rather early, as there won't be much early pressure. From there, he can move into a variety of mid-game builds, so scout appropriately and build the right counters. Be careful not to get caught with your pants down - build a small force of Zerglings, and keep Overlords on the path from your base to his, as he can still have some relatively early Zealot and Stalker pressure - nothing that you won't be able to handle with your Hydras or Mutas, though.
Ten-Gate - If you see that he has a Pylon and a Gateway but no Assimilator very early on, he may be going for an early Zealot push. The Gateway and Pylon will likely be either at the ramp to his base, or even further forward. This is countered with a few Spine Crawlers and Zerglings - Roaches if you really want to.
Two/Three Gate Robo - If you see an early Robotics Core, he is likely trying to timing attack you just as you are transitioning to Mutalisks or Hydralisks. He'll have one Pylon, an Assimilator, one Gateway, and a Cybernetics Core on the way when your Drone arrives. Again, keep him alive for as long as possible, as the Robotics Core will be dropped at around 27 supply. If you see this, tech up to Hydralisks as soon as possible - his strike force will likely consist of a bunch of Zealots, a few Sentries, and one or two Immortals. Zerglings will take the Immortals and Sentries, Hydralisks or Roaches can kite the Zealots.
*Note - There is a very uncommon build that can be deadly if you don't know what it is. It's rarely used, but I've seen it before. Basically, the Protoss player stops Drone production at 15 supply. That results in him being able to afford a second Gateway immediately after the first, at the same time as the Cybernetics Core. When you get there, you'll see a Pylon, an Assimilator, a Gateway, and a Cybernetics Core and a second Gateway building. The point of this build is to get two extremely quick Stalkers out to harass. He won't build any Zealots before, so your Drone will be able to live until you actually see the two Stalkers come out. Build two or three Spine Crawlers and some Zerglings.
Terran
Marine/Marauder - This is a timing push that a lot of Terran players will use against Zerg. When your Drone arrives, you'll likely see a Supply Depot and Barracks at the choke point. After, he'll build an Engineering Bay and a Reactor on the first Barracks, followed by two more Barracks. If I'm not mistaken, Banelings are a great way to counter this push - remember to keep macroing, though, as it isn't as much of a rush as a mid-game push.
Mech - Another annoying Terran build, I've seen tons of posts recently about how to counter. You'll know the Terran is going to Mech if you see a Factory being laid when your Drone arrives, or soon after. The key is, I think, not a certain unit, but a style of play. To counter this, you have to contain, contain, contain. He won't go for an early expansion, so you can get an Overlord and some Zerglings ready to attack when he does. The attack will consists of mostly Thors, Siege Tanks, Hellions, and maybe Vikings. Once you've got him contained, and have expanded aggressively, I would go for Mutalisks, Roaches, Zerglings, and Infestors. The Infestors will Neural Parasite the Thors. Use your Mutalisks to take out the Hellions quickly. After that, move in with the Zerglings and Roaches to take down the Siege Tanks and Thors. Make sure your Mutalisks aren't within the Thors' range when Neural Parasite wears off. Ultralisks can also be used to counter this build if you can tech up to them fast enough. Oftentimes, when this attack fails, you'll get the GG right then and there.
1/1/1 - I don't know much about this build, as I haven't seen it much. However, the opener is pretty standard. Your Drone will likely see a Barracks finished or almost finished, along with a Refinery. The Terran will then move on to building both a Starport and a Factory - this will likely lead to one of two pushes. One of them is a Hellion/Marine push, with a few Reapers possibly tossed in. Build plenty of Spine Crawlers near your mineral patches and some Zerglings - Roaches would work too, if you've got the minerals to spare. The other push he could make would be a Banshee harass - if this is the case, build an extra Queen or two and tech up to Hydralisks as quickly as possible. Build a Spore Crawler near your mineral patches in case he goes stealth. Note that no matter which push comes first, you will likely see all of these units in a 1/1/1 match - Vikings and Medivacs are also very popular. If he comes in with a Hellion/Marine push, tech up to Hydralisks quickly anyways - these will be your bread and butter in this matchup. The advantage of the 1/1/1 build is that it allows the Terran player to be flexible and build counters for anything - it actually, to be honest, allows to Terran player to play more like a Zerg. However, he won't be able to expand as fast as you. Establish total map control, and force him to build an army and make the first move. Scout appropriately and build the right counters.
Honestly, I don't really know of any other Terran builds that are used often against Zerg. If you have one, please, post it, and I'll add it to the guide.
For the final section of the guide, I'll include some Cheeses that you should be aware of. For newer players, this is especially important, as you'll likely see a lot of them in the lower brackets. A Cheese is typically an "all in" strategy, which, if defended, will leave the Cheeser severely crippled and almost guarantee a win for the defender. Don't get too frustrated when you lose to Cheese - it will probably happen eventually, and whoever did it is a noob anyway.
*Note - I haven't encountered many Cheeses, but by their nature, they can be very wacky. As such, I haven't come close to hearing about all or even most of them. If you know of one, or have one done to you, post it in here so we can raise Cheese awareness. Remember: Friends don't let friends get Cheesed.
Zerg
The most common Zerg cheese is the infamous Zergling rush. This is easy to scout out - if he's already got 4 or more Zerglings when you get to his base, you're likely about to get cheesed. The Zergling rush is very frustrating, as they can be sent in a seemingly endless stream and whittle you down until there's nothing left. Your main advantage is that he has to cross the map, and you don't. If you see this cheese coming, don't fast expand - focus on defending your main. Spine Crawlers are great, of course, and if you micro her well your Queen is also deadly. Just try to find a balance to creating just enough Zerglings of your own to defend yourself until you can tech to Roaches. Once you've got Roaches, he'll probably call the GG right there.
Protoss
Some Protoss will try to "Cannon Rush" you. This is basically when they enter your base with Probes while you can't kill them, and they'll try to build a Photon Cannon or two to end the game quickly. This can be scouted if you see a Forge before anything else - Gateways, Assimilators, probably even Pylons (he'll probably have one, but that's it). To defend this, throw down a quick Spawning Pool and have your Zerglings kill the Probes before they finish the Cannons. If you can't get a pool in time, pull your Drones (yes, every single one of them) off minerals and attack the Probes one by one.
Terran
Banshee Rush - This build may confuse you, as it can look like a 1/1/1 while he's doing it. The main difference is that with a Banshee Rush, he'll take two gasses very quickly, and likely won't have many (if any) Marines or Hellions being produced. His Starport will have a Tech Lab on it. Tech quickly to Hydralisks to easily defend this - create an Overseer or a Spore Crawler just in case he uses stealth. This is barely a cheese, as it actually takes a bit to get going - however, it is unusual and can win games if you don't know it's coming.
*Note - There are many more cheeses I would have liked to add, but I ran out of room. Check the other posts in this thread for info if you need to.
That just about wraps it up, folks. Now, this is a long post, and my fingers hurt, so I'm sure I'm forgetting some stuff. If there's anything I forgot, please chip in and I'll put it in the guide as soon as I can. Hopefully, this will help to end all the threads about scouting, and how to counter certain builds. Any questions, post and I'll try to answer them. Thanks, guys, and I hope this helps. For the Swarm!
Greetings, fellow Zerglings.
Over the past few days, there have a been a lot of topics asking why (insert Terran or Protoss strat here) keeps stomping Zerg. And among the responses, one thing is agreed upon - if you don't scout it out, you're toast. Always. So, that led to posts asking how to scout, when to scout, where to scout, such and such. However, I haven't seen any threads describing exactly what you might be seeing when you scout. So, I'm going to add that, and combine everything else into one big scouting post.
Scouting, without a doubt, is THE most crucial staple to Zerg play. It isn't Queen management, it isn't base macro, it isn't micro - it's scouting. If you don't scout, you will most likely lose. The reason behind this is that Zerg's units can be seen as highly specialized in that they excel at destroying anything they happen to be a counter to - however, they will be rolled over effortlessly if they come across a unit that counters them in the slightest. As such, if you know what units the enemy has, you're at an immediate advantage.
Here are the Zerg units that are, in my opinion, the best suited for scouting.
Zerglings - Zerglings are cheap, come in pairs, and they're fast as hell. However, they're extremely weak. Zerglings excel at scouting enemy expansions that aren't very well defended. However, any Protoss or Terran player worth his cheese will have a wall off or some kind of defense in front of his main base, so Zerglings are ineffective in that regard.
Changelings - These little guys are, in my opinion, the second most underused Zerg unit, behind Infestors. If you use them right, these guys are absolute gold. They're spawned from Overseers, and change shape into either a Marine, Zealot, or Zergling (depending on who your opponent is, of course) immediately upon enemy contact. I'll give some potential uses for these guys later on.
Drones - Drones are mainly used only as very early game scouts. Do not use them if you're anywhere past 20 supply, as they'll get killed before they can glean any useful information.
Roach - I was hesitant to put these on the list, as they're only efficient with burrow and tunneling claws. However, if you have both of those upgrades, you can burrow a roach and send him on towards the enemy army. If they have detection and he dies, it's not a huge deal, as they're cheap.
Overlord - These guys are often used by new players in all the wrong ways. I wouldn't call them scouts, even with the speed upgrade - I prefer to regard them as "observers". More about them later.
Now, here are some units that might seem good for scouting, but actually aren't.
Mutalisks - This is main reason I have this part of the guide. I see a lot of new players using Mutalisks to scout, as they're fast and can fly. However, they're extremely expensive, and can be put to much, much better uses harassing the enemy. Don't ever sacrifice these guys.
Banelings - This is self explanatory. Just use the Zergling you morphed it from and save yourself the gas.
Hydralisks - A lot of newer players may use these to scout because they are often the staple of a Zerg army, and they figure that the loss of only one is insignificant. Don't waste the gas when these can be much more efficiently used in an army.
Now that I've listed what units can scout, I'll describe exactly how they should go about doing so.
Zerglings - Like I said earlier, Zerglings are best used for scouting expansions. They won't make it into a main base before being killed. They are doubly effective at scouting expansions as if you can catch one early, it's easy to take it out.
Changelings - These are your main base scouters. However, like Zerglings, you can't send them through the front door. Next time you play a game, take a moment to look at your main base. There's probably a big cliff, or drop off that cannot be traversed, along the outside edge of the map. Naturally, you don't build anything there, right? Well, neither do Terran or Protoss players. So, take one of your Overlords (you should have plenty) and morph him to an Overseer. Let him hang out near the blind spot of your enemy's base, outside his field of vision. When you want to know what's up, move your Overseer right to the edge of the base, drop a Changeling, and get back out of sight. That Changeling will probably have free reign of the base, but if you're spotted and killed, you were probably close enough to his army to see what he was making anyways. Even if you only see one Banshee or one Colossus, remember that nobody just makes one of a unit and then forgets about them. If you see one of them, it's safe to assume there were more in the fog.
Drones - You will typically only send one Drone per game to scout. Depending on whether you build your Overlord at 9 or 10 supply, you'll send your 9th or 10th Drone to scout. (If 9 Ovie, send 10th Drone, if 10th Ovie, send 9th Drone. Duh.) Go ahead and send him right on in to your opponent's base, as he won't have anything that can kill it quite yet. Send him on a wild course of shift-clicking through the base. Make sure not to focus all of your attention on keeping him alive; keep your macro going, as you'll probably be expanding around this time. When the opponent builds a Marine or Zealot to attack your Drone, I like to send him to the nearest Vespene source and build an Extractor. Keep an eye on the Extractor - just as it finishes, cancel it, and you'll be targeting the Drone again by default. So, you cancel, then press "B" and "E" quickly, and build another. This is extremely annoying, and can often effectively neutralize a lot of annoying openers. Plus, less experienced players may panic and devote far too many units to killing your Extractor.
*Note that the information you gather with this Drone will likely be the most important information you glean for the entire game - later in the guide, I'll go over common Terran and Protoss build orders so you know what you're seeing.
Overlords - Possibly the most common mistake of new players is to send their initial Overlord directly to your opponents base. Do not do this. If you scout with your 9th or 10th Drone, he beats the Overlord there. By the time the Overlord gets there, a Terran player will have a Marine to shoot him down or a Protoss player will make a Stalker to shoot him down. Instead, send your first Overlord to your opponent's natural expansion. Send your second to the next closest expansion. Keep doing this until you've covered every mineral patch on the map. This is one of the most effective containment strategies you can use - undoubtedly, during midgame, you'll have a group of around 10 Zerglings that are just chilling, doing nothing. Keep them ready to strike - when one of your Overlords sees the enemy going for an expansion, move in. Target his SCV or Probe first, then take down the Nexus/Command Center. Your opponent just effectively wasted 450 minerals (if he didn't cancel - he probably will) and a good bit of time, and you still have an economic advantage (assuming you expanded early like most good Zerglings should). Another good idea is to keep an Overlord stationed far to the left of your opponent's base, and another far to the right. If you ever direly need to scout, send them both in at once, and you'll see something. Don't be afraid, in the late game, to send your Overlords on a suicide mission to scout, if that's your only option. They're cheap to replace, and if you have the speed upgrade (which you NEED TO HAVE) you'll likely get the information you need.
If any of you are wondering when you should be scouting, the answer is simple. Always. You don't have to be scouting every second of the game, but it should be one of the closest things to the front of your head as you play. Remember, it only takes about a minute and a half to build 6 or 7 units that you totally aren't prepared to face.
Now, for the final, and arguably most important, part of the guide. If you've been reading the forums, you should know that you always need to send a Drone at the beginning to scout. However, newer players that have only played Zerg likely won't even know what they're seeing when they see it. To finish up, I'm going to list some common Protoss and Terran openings that can be very annoying if not properly scouted.
Protoss
The Four-Gate - This is one of the most notoriously annoying Protoss openers, and absolutely deadly if you don't scout it and react quickly. In most games, your Drone will reach your opponents base when you have about 16 supply. At 16 supply, a Protoss going for a Four-Gate will probably have one Pylon, one Gateway, one Assimilator, and a Cybernetics Core on the way. This doesn't necessarily mean they're going for a Four-Gate, it could also be a Void Ray rush. As such, it's important to stick around with your one Drone for as long as possible when fighting a Protoss opponent. If he throws down a second Gateway, it's a Four-Gate. If it's a Four-Gate, he will also be repeatedly Chrono-Boosting his Cybernetics Core, researching the Warp Gate upgrade. When you see this happen, build a Roach Warren as soon as possible, and put up at least three Spine Crawlers at your natural. Build a healthy force of Zerglings with speed as well, and you should have no problem fending off this rush. Make sure you scout the area near your base and remove any proxy Pylons he may have laid down.
The Void Ray Rush - This opener is extremely deadly if you aren't prepared for it. If you don't see it coming, it will almost certainly lead to the GG. At 16 supply, a Void rush will actually be identical to the Four-Gate, probably - one Pylon, one Gateway, one Assimilator, and a Cybernetics Core. If he throws down a Stargate after the Cybernetics Core, it's Void Rays for sure - he might also throw down a second Assimilator. Even if you see this coming, it's sometimes a pain to deal with. Build a few extra Queens, and tech up to Hydralisks as quickly as possible. If you macro well, you should have 6-7 Hydralisks out by the time his first Void Rays even reach your base, and you should be good from there on out. Note that an early Stargate could also mean he is going for a Phoenix harass. This isn't as threatening to your base, however - if you don't kill Phoenixes, they will severely cripple your economy. Use the same counters that you would use against Void Rays - Queens are very effective against Phoenixes.
Fast Expand Builds - The Protoss typically won't go for these. If they do, however, you'll likely see a quick Forge, so they can throw down Photon Cannons immediately to protect their expansion. The best way to counter this is to take a third expansion rather early, as there won't be much early pressure. From there, he can move into a variety of mid-game builds, so scout appropriately and build the right counters. Be careful not to get caught with your pants down - build a small force of Zerglings, and keep Overlords on the path from your base to his, as he can still have some relatively early Zealot and Stalker pressure - nothing that you won't be able to handle with your Hydras or Mutas, though.
Ten-Gate - If you see that he has a Pylon and a Gateway but no Assimilator very early on, he may be going for an early Zealot push. The Gateway and Pylon will likely be either at the ramp to his base, or even further forward. This is countered with a few Spine Crawlers and Zerglings - Roaches if you really want to.
Two/Three Gate Robo - If you see an early Robotics Core, he is likely trying to timing attack you just as you are transitioning to Mutalisks or Hydralisks. He'll have one Pylon, an Assimilator, one Gateway, and a Cybernetics Core on the way when your Drone arrives. Again, keep him alive for as long as possible, as the Robotics Core will be dropped at around 27 supply. If you see this, tech up to Hydralisks as soon as possible - his strike force will likely consist of a bunch of Zealots, a few Sentries, and one or two Immortals. Zerglings will take the Immortals and Sentries, Hydralisks or Roaches can kite the Zealots.
*Note - There is a very uncommon build that can be deadly if you don't know what it is. It's rarely used, but I've seen it before. Basically, the Protoss player stops Drone production at 15 supply. That results in him being able to afford a second Gateway immediately after the first, at the same time as the Cybernetics Core. When you get there, you'll see a Pylon, an Assimilator, a Gateway, and a Cybernetics Core and a second Gateway building. The point of this build is to get two extremely quick Stalkers out to harass. He won't build any Zealots before, so your Drone will be able to live until you actually see the two Stalkers come out. Build two or three Spine Crawlers and some Zerglings.
Terran
Marine/Marauder - This is a timing push that a lot of Terran players will use against Zerg. When your Drone arrives, you'll likely see a Supply Depot and Barracks at the choke point. After, he'll build an Engineering Bay and a Reactor on the first Barracks, followed by two more Barracks. If I'm not mistaken, Banelings are a great way to counter this push - remember to keep macroing, though, as it isn't as much of a rush as a mid-game push.
Mech - Another annoying Terran build, I've seen tons of posts recently about how to counter. You'll know the Terran is going to Mech if you see a Factory being laid when your Drone arrives, or soon after. The key is, I think, not a certain unit, but a style of play. To counter this, you have to contain, contain, contain. He won't go for an early expansion, so you can get an Overlord and some Zerglings ready to attack when he does. The attack will consists of mostly Thors, Siege Tanks, Hellions, and maybe Vikings. Once you've got him contained, and have expanded aggressively, I would go for Mutalisks, Roaches, Zerglings, and Infestors. The Infestors will Neural Parasite the Thors. Use your Mutalisks to take out the Hellions quickly. After that, move in with the Zerglings and Roaches to take down the Siege Tanks and Thors. Make sure your Mutalisks aren't within the Thors' range when Neural Parasite wears off. Ultralisks can also be used to counter this build if you can tech up to them fast enough. Oftentimes, when this attack fails, you'll get the GG right then and there.
1/1/1 - I don't know much about this build, as I haven't seen it much. However, the opener is pretty standard. Your Drone will likely see a Barracks finished or almost finished, along with a Refinery. The Terran will then move on to building both a Starport and a Factory - this will likely lead to one of two pushes. One of them is a Hellion/Marine push, with a few Reapers possibly tossed in. Build plenty of Spine Crawlers near your mineral patches and some Zerglings - Roaches would work too, if you've got the minerals to spare. The other push he could make would be a Banshee harass - if this is the case, build an extra Queen or two and tech up to Hydralisks as quickly as possible. Build a Spore Crawler near your mineral patches in case he goes stealth. Note that no matter which push comes first, you will likely see all of these units in a 1/1/1 match - Vikings and Medivacs are also very popular. If he comes in with a Hellion/Marine push, tech up to Hydralisks quickly anyways - these will be your bread and butter in this matchup. The advantage of the 1/1/1 build is that it allows the Terran player to be flexible and build counters for anything - it actually, to be honest, allows to Terran player to play more like a Zerg. However, he won't be able to expand as fast as you. Establish total map control, and force him to build an army and make the first move. Scout appropriately and build the right counters.
Honestly, I don't really know of any other Terran builds that are used often against Zerg. If you have one, please, post it, and I'll add it to the guide.
For the final section of the guide, I'll include some Cheeses that you should be aware of. For newer players, this is especially important, as you'll likely see a lot of them in the lower brackets. A Cheese is typically an "all in" strategy, which, if defended, will leave the Cheeser severely crippled and almost guarantee a win for the defender. Don't get too frustrated when you lose to Cheese - it will probably happen eventually, and whoever did it is a noob anyway.
*Note - I haven't encountered many Cheeses, but by their nature, they can be very wacky. As such, I haven't come close to hearing about all or even most of them. If you know of one, or have one done to you, post it in here so we can raise Cheese awareness. Remember: Friends don't let friends get Cheesed.
Zerg
The most common Zerg cheese is the infamous Zergling rush. This is easy to scout out - if he's already got 4 or more Zerglings when you get to his base, you're likely about to get cheesed. The Zergling rush is very frustrating, as they can be sent in a seemingly endless stream and whittle you down until there's nothing left. Your main advantage is that he has to cross the map, and you don't. If you see this cheese coming, don't fast expand - focus on defending your main. Spine Crawlers are great, of course, and if you micro her well your Queen is also deadly. Just try to find a balance to creating just enough Zerglings of your own to defend yourself until you can tech to Roaches. Once you've got Roaches, he'll probably call the GG right there.
Protoss
Some Protoss will try to "Cannon Rush" you. This is basically when they enter your base with Probes while you can't kill them, and they'll try to build a Photon Cannon or two to end the game quickly. This can be scouted if you see a Forge before anything else - Gateways, Assimilators, probably even Pylons (he'll probably have one, but that's it). To defend this, throw down a quick Spawning Pool and have your Zerglings kill the Probes before they finish the Cannons. If you can't get a pool in time, pull your Drones (yes, every single one of them) off minerals and attack the Probes one by one.
Terran
Banshee Rush - This build may confuse you, as it can look like a 1/1/1 while he's doing it. The main difference is that with a Banshee Rush, he'll take two gasses very quickly, and likely won't have many (if any) Marines or Hellions being produced. His Starport will have a Tech Lab on it. Tech quickly to Hydralisks to easily defend this - create an Overseer or a Spore Crawler just in case he uses stealth. This is barely a cheese, as it actually takes a bit to get going - however, it is unusual and can win games if you don't know it's coming.
*Note - There are many more cheeses I would have liked to add, but I ran out of room. Check the other posts in this thread for info if you need to.
That just about wraps it up, folks. Now, this is a long post, and my fingers hurt, so I'm sure I'm forgetting some stuff. If there's anything I forgot, please chip in and I'll put it in the guide as soon as I can. Hopefully, this will help to end all the threads about scouting, and how to counter certain builds. Any questions, post and I'll try to answer them. Thanks, guys, and I hope this helps. For the Swarm!