During gameplay, press enter and you can see two dotes there flashing, then you type those codes:

POWER OVERWHELMING - Godmode
SHOW ME THE MONEY - Give you minerals and gas
OPERATION CWAL - Speeds construction of buildings and units
FOOD FOR THOUGHT - Ability to build units beyond the support limit
THE GATHERING - Gives unlimited energy to all casing units
SOMETHING FOR NOTHING - Gives all upgrades
MODIFY THE PHASE VARIANCE - Ability to build all buildings
BLACK SHEEP WALL - Shows entire map
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
MEDIEVAL MAN - Free upgrades to units
WAR AINT WHAT IT USED TO BE - Disables fog of war
OPHELIA - Enter this to enable level skipping cheat. Then enter the mission you want to skip to