Here are the most generaly used cheats... They will work for any version of Duke Nukem 3D!

DNKROZ-Gives invincibility!
DNSTUFF-Gives all guns,ammo,keycards and items!
DNCLIP-Gives you the ability to walk through walls! (Can sometimes crash the game)
DNMONSTERS-Makes all the monsters dissapear!

(These are the most commonly used codes)