Quick Reaper

The quick Reaper build is to harass the enemy and kill as many workers as possible. It is most effective against Protoss, but might be used against Zerg. You really have to make sure to damage the economy, as the quick reaper will force you to cut SCV-production in the beginning.

You want to make your Barracks and Refinery early, before your supply depot. You have to start harvesting gas at once, but you might consider pulling the SCVs off gas if you stop producing Reapers. As soon as the Barracks is finished, start your Tech Lab. The Reaper is built at once when the Tech Lab is finished. You can choose if you want to build your Orbital Command or not, but it is recommended because you are low on SCVs harvesting minerals, and the MULEs will help a lot.

Your early Reapers will usually not win you the game, but if you are able to take out a lot of the workers, you are in good shape. Make sure your Reapers don’t die, and keep harassing. This build is microintensive, and you have to make sure you are not forgetting transitioning into another Tech-Lab build. A typical transition is into tripple barracks with an expansion. Build your second Command Center inside your base and make an Orbital Command before moving out. When your Orbital is finished, you usually have transformed into a Marine/Marauder-army, and are able to defend your natural expansion.