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.