Contrary to the other submission, you cannot open up the console by just pressing the "~" key; to actually open the console you must press the following at the same time:
CTRL + ALT + ~
After doing so, enter one of the following codes:
NOTE: At the beginning of a new level the codes will automatically be turned off, be ready to do so again
god (God mode)
give keys (Aquire all of the keys you'll need throughout the game)
give all (All weapons with full ammo, health and armor)
give berserk (Berserk mode)
status (Give game status)
doomhell (Skip to last level)
give health
give ammo