Enter these during gameplay in doom. Dont press enter. Cheats dont work on nightmare mode and in any kind of network play.

iddqd - godmode
idkfa - all guns, ammo, keys
idfa - all guns, ammo
idspispopd - no clipping, walk though walls
idbehold - choose from a menu (by pressing the 1st letter)
idchoppers - chainsaw
idmypos - your coords in HEX
iddt - works in automap, shows all map
iddt - (again) shows all monsters idclev(EL) - change level, where E is episode, L is level

Also see: Doom Command Line Parameters