Press Tab while you are in the game to bring up the console. This will pause the game and will display the prompt: At this prompt, type one of the console Commands, then press ENTER.

Effect	Password
Allow use of all five weapons	iwantallweapons
Cancel freeze mode (see above)	stopfreeze
Fly	icanflytoo
Freeze ducks [allows multi-kill of same duck ;)]	freeze
No Effect	noduckhair
Permanent 100% health	givemehealth
Show internal game info, used for debugging	showdebuginfo
Unlimited ammo for all available weapons	unlimitedammo

Note: Using any cheat code means that you cannot save a high score for that game.