Here are some cheats that will only work when you are playing one player(no multi player). The way they work is you push enter when your game has started. You should get the message bar. Now type in the cheat. You need to put in spaces were i have.

Power overwhelming - God Mode

Show me the money - Gives you 10,000 gas and 10,000 crystal

Operation CWAL - Speeds construction of buildings and units

The Gathering - Gives unlimited energy to all casting units

Game over man - Ends your game as a loss

Staying Alive - Prevents the mission from ending due to victory or defeat

There is no cow level - Completes the current mission

Whats mine is mine - Free minerals

Breathe deep - Free Vespene Gas

Something for nothing - Gives all upgrades

Black Sheep Wall - Shows entire map

Medieval man - Free upgrades to units

Modify the phase variance - Ability to build all buildings

War aint what it used to be - Disables fog of war

Food for thought - Ability to build units beyond the support limit

Ophelia - Enter this to enable level skipping cheat. Then enter the mission you want to skip to (i.e., "terran10") to go there.