Once the game starts, hit the ~ key and type one of the following codes. Note that some codes only work for certain animals.

GOD - god mode
LIGHT - creates a light source near player
FREAKOFTHEUNIVERSE - god mode for Marine
SKULLCOLLECTOR - god mode for Predator
THEONEDEADLYCREATUREEVERCREATED - god mode for Alien
GIMME_CHARGE - replenishes the Predator's energy
WINNEROFTHEONEGREATBATTLEOFTHEUNIVERSE - turns the Alien into the final Xenomorph
GIVEALLWEAPONS - gives you all weapons and ammunition
OBSERVER - turns you invisible to all enemies
SHOWPOLYCOUNT - show number of polygons
SHOWFPS - gives you a frame rate count
LISTCMD - list commands