You just have to type the following magic words while playing:

iddqd			Degreelessness mode (God mode)
idkfa			Very Happy Ammo. (full ammo + 200% armour, no backpack)
idspispopd		No clipping (SPISPOPD?)
idbehold		followed by S, V, I, R, A, L for various things (menu)
idclev			followed by episode number and level number: warp
idmypos			prints your co-ordinates in hex
idchoppers		prints 'Doesn't suck - GM' (Gives you chainsaw)
iddt			increase map detail (do it multiple times in map mode)

Elias.
ep104@cus.cam.ac.uk.