During gameplay, type:

 elvis    - Unlimited health
 help     - Pauses the game
 tools    - All weapons
 mmmmungo - Your hero gets stronger
 billy    - Your hero gets faster
 caffeine - Healing
 taxi     - Next level

 And some editing:

 Edit the config.dat file in the Deathtrap Dungeon\Asylum
 directory. Change the PROGRESS line to:

 PROGRESS 1023

 This will enable you to play all the levels in the Single
 Player Game menu.