Start the game with the -cheats -funlabs command line parameter to enable cheat mode. Then, press ~ during game play to display the console window. Note: There is a space before and after the equals sign "=" when it appears in a code.

Code - Result:

godMode = [0 or 1] - God Mode
noClip = [0 or 1] - No Clipping
give health - Add Health
give weapons - Add Weapons
aiDisableThink = [0 or 1] - Disable AI Thinking
aiDisableAct = [0 or 1] - Disable AI Acting
revNextSpawn - Jump to Next Spawn Point
spMapList - List Maps
help - List All Commands