To gain access to these cheats hit ENTER, TYPE IN THE CHEAT, and then hit ENTER once again. After you complete these steps the cheat should then be enabled.

Ophelia = Enter this to enable level skipping cheat. Then enter the missionyou want to skip to (that is, "terran10") to go there.
Breathe deep = Free Vespene Gas.
Medieval man = Free upgrades to units.
Game over man = Ends your game as a loss.
The Gathering = Gives unlimited energy to all casting units.
Staying Alive = Prevents the mission from ending due to victory or defeat.
Operation CWAL = Speeds construction of buildings and units.
Black Sheep Wall = Shows entire map.
Food for thought = Ability to build units beyond the support limit.
Show Me The Money = 10000 in ore and gas.
Show me the money = Gives you 10,000 gas and 10,000 crystal.
Power overwhelming = God Mode.
Whats mine is mine = Free minerals.
Power Overwhelming = Can't be beaten.
There is no cow level = Completes the current mission.
Something for nothing = Gives all upgrades.
Modify the phase variance = Ability to build all buildings.
War aint what it used to be = Disables fog of war.

Note:
All codes only work in Single Play mode ONLY