Type any of these cheats at the console for their effects.
(note: to access the console press the tilde key during play

Weapons and Ammo cheats:

give all
give shotgun
give shells
give sshotgun
give machinegun
give bullets
give grenade launcher
give grenades
give rocket launcher
give rockets
 

For health type: give health
 

For invincibility type: god
 

For no clipping type: no clip