This strategy can be very effective against opponents who build big units like Battlecruisers or Carriers.

-Build 7 SCVs, make 7th build a supply depot.
-Build more SCVs, get 4 Barracks, get a Refinery.
-After 4 Barracks are done, start pumping out Marines, and get out an Academy.
-Build an Engeniering Bay or two, and research U-238 shells and Stimpack.
Note: Depending on the game, you might want to build defense first.
-Build a Factory, and then 2 Starports with Control Towers.
-While doing all this, upgrade Infantry Weapons and keep pumping out tons of marines.
-Research Wraith cloaking and energy, and get out some dropships.
-Do that until you can get at LEAST 7 or 8 dropships.
-Build about 12 Wariths.
-Put a bunch of back-up Marines in your base.
-Look for a place in your opponent's base that is not heavily guarded, and bust in.
-Cloak your Wraiths and attack all their defense.
-While Wraiths are busy, drop your marines in an unguarded area, like in the middle of their workers or a bunch of buildings.
Note: If you want, you can put 6 marines and 2 Medics in each Dropship, rather than just 8 marines. I don't find Medics that helpful late in the game, but hey, every game is different.
-This should kill your opponent.
-If it doesn't, he/she will probably attack, and since you got back-up Marines, it shouldn't be a problem.
-Then attack him/her again.