Build Order Counters

Scouting is One of the most important actions of Starcraft 2.
Starcraft 2 generally revolves around countering your opponents units with a better mix of units.
As there are numerous builds that the 3 races can do, I will post these in segments.

Since I am a mainly Zerg player, I will start with Zerg counters.
Against terran, you will either see a bio ball, or go straight to mechs. If I am scouting early and before they completely wall off their base, and see a tech-lab, they are most likely going for marauders or reapers (duh).
This is very dangerous for a zerg player as Zerg needs to be able to expand quickly to be able to swarm the enemy. Try to keep the drone inside their base as long as possible to see what their first unit is.

1) if its a reaper, i would instantly get some spine crawlers around my mineral line (3-5). I would also research roach speed as soon as possible and get roaches. This is because reapers eat through zerglings like well zerglings. With this defence, you would be able to defend most spots, until you can get hydralisks or mutalisks.

2) If its a marauder, the obvious answer is banelings. Get centrifugal hooks as soon as possible as this greatly increases their speed. Back them up with as much zerglings as possible, and brace yourself for their push.
If your feeling pretty confident, (you scouted their base succesfully with an overlord or a zergling and they dont seem to have a huge army) Go for that baneling bust. Send in your banelings to destroy the supply depot or bunker and then tear through the rest of the army with your speedlings. Please note that it only takes around 5 banelings to destroy a supply depot. Keep the rest to kill off the infantry inside or behind it.

If you chose option 1, then life is going to get hard for you. Chances are, that the opponent will instantly transition from reapers to a MMM army. If you were able to defend the harrass well with almost no casualties, youre economy would be in a great shape provided that you had an early expansion. I would suggest overseers as soon as possible. Not only are they the only detectors zerg have, but they have an invaluable skill, called spawn changeling. You have no idea how many games I won because I was able to drop a changeling in their base and scout what they were doing.

Hope this helped and I will be posting more stuff on this later as there are many many strategies to deal with.