During play, press ~ to bring up the console and enter the following.

pkgod	God mode on/off
pkweapons	All weapons
pkammo	Full ammo
pkpower	Full health and ammo
pkgold	More gold
pkkeepbodies	Toggle bodies
pkkeepdecals	Toggle decals
pkdemon	Toggle demon morph
pkweakenemies	Toggle weak enemies
pkalwaysgib	Toggle excessive gibs
pkhaste	Toggle haste

Thanks to Revolution readers Chief and duckman!