HERETIC 2 DEMO
Activision
Go to the console by pressing the '~' key, then type in the following: 
Playbetter	Toggle God mode on/off
twoweeks	Toggle infinite power-ups on/off
aquaticape	No target on/off 
kiwi	Clipping on/off
angermonsters	Monsters get a bit testy
crazymonsters	Monsters go absolutely berserk
victor	Kills all monsters
Try out other codes too. Games of this type have pretty basic rules when it comes to cheating, and you can easily stumble across new ones.