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

Code - Result:
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)