Pincer Drop

This strategy is tricky to pull off, but can be very rewarding.
Note: Not recommended for 2v2 or 3v3 games. Unless your partner(s) are willing to defend you, you probably shouldn't do this.
BUILD ORDER:
-First start off with your regular build order. I suggest a Depot on the 8th SCV followed by a Refinery on the 9th. Gas is very important for this strategy.
-Now build 2 Barracks while pumping SCVs for minerals. Get 2 SCVs on gas and the others on minerals. Begin pumping Marines.
-Once the 2 Barracks are done building take the 2 SCVs and build 2 Depots. While this is being done build another SCV. When the Depots are done put 1 SCV to make an Academy and take the other SCV and the one you just built to make 2 Factories. (Your cash flow may not permit this, so just wait for the minerals then build.)
-You should have 6-12 Marines now, depending on your cash. Once the Academy is done build a Comsat, 2 Medics and another Barracks. If opponent is Zerg, or you think they are going air, also build an Armory.
- Build 1-3 more Bunkers and get 2-4 groups of Medarines (11 Marines per 1 Medic). Your Factories should just be finished. Make a Starport and get 2 Dropships ASAP. Fill the Dropships with 4 Vultures and whatever other units you'd like. (If the opponent is Zerg I suggest 4 Vultures 1 Tank [Siege Mode researched], and 2 Goliaths to hunt the Overlords)

ATTACK SEQUENCE:
!!!NOTE:!!! (VERY IMPORTANT) Once you are attacking take time to queqe up some units for a second wave. In MOST CASES the first attack dies off, but leaves the enemy severly crippled. I have only once won on the first stike, SO GET REINFORCEMENTS READY!!! Also, this is the time to build a few Turrets and Bunkers for Defense.
-Move your Medarines in front of your opponents base (you should have scanned with the Comsat to find) and camp for awhile. If minerals permit send down some reinforcements and a Vulture or 2 to 'Mine Off' the entrance so no workers escape.
-Position your Dropships near the edge of your opponents base and prepare to drop. If they are Protoss or Terran I drop first, then attack. If they are Zerg I attack and then Drop. Protoss and Terran have better counters for the Medarines so I like to draw their units away.
-Your infantry should be hacking away (so to speak) at your opponents front while your DropSquad are demolishing their workers. For more defense use your dropped Vultures to mine by the Command Center/Nexus/Hatchery so fewer units can back up their workers.
-Now move in your second wave. It should be about 2 Medarine Groups, 2-6 Tanks, 2-8 Goliaths, and some Wraith backup. Have the tanks do most of the work. If they counter with air, your Wraiths and Goliaths usually prevail. Your Medarines and Goliaths should ward of any feeble attempts to assault your backup.

VARIATIONS:
-Firebats work wonderful instead of Vultures, but I always liked using the mines to ward off offending Zerglings, Zealots, and other units that tend to object to your killing of their workers.

-Sending backup via Dropship is also a good idea. After your drop of Vultures/Tanks/Goliaths a followup of 16 Medarines wouldn't hurt ;-)
-Sci Vessels are a bit more dependable than your Comsat. I suggest getting them directly after the attack.

I am not an expert at this strategy, but everytime I use it I at least weaken the enemy. If your second wave fails, its best to turtle in and Tech up to Battle Cruisers. If that doesn't work then you're ska-rewed. Good Luck