In the the game, press ~ to enable the console window on the insomnia or daydream difficulty setting, then enter one of the following codes:
Toggle God mode: pkgod
All weapons: pkweapons
Full ammunition: pkammo
Full ammunition and health: pkpower
Extra gold: pkgold
Haste: pkhaste
Disappearing bodies: pkkeepbodies
Disappearing decals: pkkeepdecals
Demon morph: pkdemon
Weak enemies: pkweakenemies
Excessive gibs: pkalwaysgib
Weapon modifier mode: pkweaponmodifier