While playing, press ^ (tilde) to bring up the console then type any of the following codes: 

NITRO GOD          :: God Mode 
NITRO GIVEALL All  :: Weapons/Ammo 
NITRO GIVEMONEY    :: Add Cash 
NITRO KILLALL      :: Kill All Enemies 
NITRO FLY          :: Fly Mode 
NITRO GHOST        :: Ghost Mode 
NITRO REFRESH      :: Full Health 
NITRO CHANGELEVEL  :: Skip Level 
NITRO SPEED X      :: Increase Speed to X (1-10)