MDK

To use these codes, hit the [F1] key to get to the help screen first.
Most of these codes can only be used once per level.

American Version:
MAKEMEFULL Full health
MASTERBLASTER Super chain gun powerup
TWISTANDSHOUT Tornado item
BIGGRENADE Homing sniper grenade ammo
KILL Kill yourself

European Version:
HEALME Full health
INEEDABIGGUN Super chain gun powerup
HOLOKURTISFUN Dummy item
TORNADOAWAY Torando item
NASTYSHOTTHANKS Homing sniper grenade ammo
KILL Kill yourself

To use this code, just type it in after the drop from space on the first level:
SHOWMOVIEMDKBZK Play hidden movie
(Information from Cheat Machine)