In this guide I will tell you how to effectively get 3 reapers out with nitro packs within 3 minutes of the game. This is very effective vs. early zerg and can be used for worker harass. It can also be very good against protoss unless they scout it and get an early stalker.
The idea is to consistently make SCVs nonstop. If you follow the build correctly you will make them nonstop up until the point of researching orbital command.
At 10 supply make a supply depot at your choke. The next 150 minerals you get make a barracks, and as soon as you can after, get a refinery. Remember to always have one SCV building while doing this. Never make a queue as this will waste minerals. Watch your refinery very carefully, as SOON as it finishes put two extra SCVs on it. Make another depot with extra minerals. As soon as the Barracks finishes you should be able to start the tech lab with the 25 gas you should already have. Start researching orbital command and let your money build up.
As soon as the add-on is complete, start making a reaper and start researching nitro packs at the same time. Continue to make two more reapers. By the time nitro packs is done, you should have 3 reapers, now you can start your harass with the extra speed. Take out workers and take out any singled out pylons. Use a scan before hand to check for any weaknesses.
Remember to start making additional barracks or tech up to factory depending on what build you're going while doing this.
I recommend marauders with slowing shells against protoss because of the tendency for them to mass stalkers after seeing reapers.
Against zerg, mass marines with rax w/ reactors and start teching to tanks w/ seige and grab a starport with reactor to start getting medivacs. If you see roaches, get more marauders or even go banshees. Keep scanning and get the counter units.