To activate cheats, first press the tilde (~) key to bring up the console. Then, type in
the code and press enter. Press escape to get back to the action.
god-god mode
notarget-monsters won't attack if not provoked
noclip-no clipping mode
give all-gives all items and weapons
The rest of these are pretty self-explanatory:
give jacketarmor
give blaster
give shotgun
give sshotgun
give machinegun
give grenadelauncher
give rocketlauncher
give shells
give bullets
give cells
give grenades
give rockets
give slugs
give mines
give nuke
give quad
give invulnerability
give silencer
give rebreather