During gameplay, press T to bring up the talk screen, and enter any of the following

Code	Effect
armor	Restores full armor
health	Restores full health
god	Turns God mode on or off
poltergeist	Turns invisibility on or off
gimmegun < weaponname >	Type in the name of the weapon you wish your character to have
ammo	Full ammunition
guns	All weapons
gear	More Health, Reflex and 10 Med Packs
pos	Position mode
build	Build mode