This is a pretty wacky strategy, but it will pretty much ensure invincibility against the Protoss, Zerg, or Humans (pending the amount of Tanks the opponent has against you). So, here goes. This only works on certain levels

Start out by working all of your SCV's to begin with; while doing that, create another. Keep on doing this until you are at 8 SCV's, and make one supply depot behind your base. Once you have enough minerals, blockade all the corridor entrances on your base with supply depots. The supply depots will serve as a barriar against any opponents' attacks. Safeguard the barrier with bunkers behind every supply depot, and a tank in Siege mode between every bunker. Fill the bunker with marines, and station about 10 marines behind all of them without protection, scattered. Make sure you've researched the extended range shells at the academy before you enact this, also. You want to upgrade as much as you can while being as cost effiecent as possble.

This could be pretty pricy, and you'll probably need at least 2 command centers with 25 SCV's working. In addition to this, make sure that you outline every corner of your base with a turret, and don't be shy to make marines! Marines are some of the more effective tools to the terran attack as they can attack both land and air. But if you have the time and the patience, this strategy is very effective to people looking to devistate you with raw power.

CreepColony's Opinion: This is a good defense against comps, but not really against humans. Most humans will go all air (BCs, Carriers, etc) or mass ground (hydras, goons), and in that case, your defense should just be a ton of tanks and several missile turrets.