If you complete level 9 of the Zerg campaign in less than 20 minutes you can unlock a secret level. To access the level again make sure you save the game on that stage, as it won't appear on the in-game stage list.
You can also give these cheats a whirl. During the game press Enter, type a code and hit Enter again to activate it.
THERE IS NO COW LEVEL Mission skip
OPHELIA Enable mission select cheats (type this before entering the next three cheats)
TERRANx Terran mission select, where x is the mission to select
ZERGx Zerg mission select, where x is the mission to select
PROTOSSx Protoss mission select, where x is the mission to select
BLACK SHEEP WALL Show whole map
WAR AINT WHAT IT USED TO BE Turn off fog of war
MAN OVER GAME You win
GAME OVER MAN You lose
SOMETHING FOR NOTHING Upgrade everything
MEDIEVAL MAN Free upgrades for units
OPERATION CWAL Fast building and instant upgrades
THE GATHERING Limitless psionic ability
POWER OVERWHELMING Invincible buildings and ships
FOOD FOR THOUGHT Ignore supply limit when building units
NOGLUES Opponent has no psionics
MODIFY THE PHASE VARIANCE Build anything you want
SHOW ME THE MONEY 10,000 added to minerals and gas
WHATS MINE IS MINE Increase minerals by 500
BREATHE DEEP Increase gas by 500
STAYING ALIVE Continue after mission has been completed
RADIO FREE ZERG Zerg song (available only while playing as Zerg)
