Various cheats

     Press  ~  (top  left  of keyboard directly below the escape key),
sometimes called tilde during the game.

Cheat code:             Effect:
GodMode()               invincibility 
Cheat.FullHealth()      full health 
Cheat.TuneWeapons()     all weapons 
Cheat.AddAmmo()         100% ammo 
Cheat.ClearOverload()   unsure 
Cheat.NextMission().    self-explanatory.