Weapon cheats
Hit F1 (help screen) before typing these in, then hit Esc or Enter to get back to the action. They can only be used once per level. Bear in mind that there are various patches and versions of MDK floating around, so you could find that these cheats don't work. 

Ineedabiggun	Super chaingun power-up
Holokurtisfun	Dummy power-up
Nastyshotthanks	Homing sniper grenade
Tornadoaway	Twister power-up
Iliketolob	Mortar power-up
Kill	Suicide!
486ok	Allow 486 machines to run