8 Marine SCV Rush
This (almost always) all-in build allows for a hard hitting rush that usually catches opponents off guard. This build allows for a timed push and the ability to constantly stream in Marine and SCV reinforcements.
The objective of this build is to defeat your opponent in a single hit or damage his economy so severely that your opponent cannot continue. The pros of this build are that it allows for a fast win versus a greedy or teching opponent. The cons are that it is very usually all-in, and susceptible to Roach builds and fast tank/mech builds.
Basic Build Order
After the building of the first Barracks, immediately queue up a Marine and an Orbital Command. Remember to continuously build Marines from both Barracks and constantly use MULEs. After you build 8 Marines, hotkey them to a group, take all your SCVs off and hotkey them to a separate group (hitto return cargo before you go), set your Barracks also on a hotkey with the rally to the enemy base. After all this is complete, start the assault while making sure that your SCVs are in the front so that they will tank for your Marines. Remember to continue calling down MULEs and producing Marines to reinforce your attack. After you take out your enemy's initial force, immediately attack their workers. If you feel that the push did not work, immediately reset your Barracks rally point and call back any units you have out. Be ready to switch into a standard mid-game build (preferably one with less teching) and start making SCVs.
When scouting after the initial Supply Depot, if against a Zerg, always check for 6-pool. It is recommended that you re-use your scout to check for Roaches (if you scout this, stop your attack, it will not work, trust me). If against Terran, check for fast Tanks (this could be a problem depending on when they get the Tank out), and if against Protoss, check for 2-gate Zealot pushes (usually you will be able to take this on).
For adaptations and what to do against certain unit mixes, please see the Execution and Scouting sections above.
Steppes of War or preferably maps with short rush distance or 1v1 maps.
Short distance between bases and large ramps.
Long distances between bases.