God Mode
While playing, press SHIFT and TILDE (~) to enter the menu, then type "juggernaut" to activate god mode. To disable god mode, reenter the code.
Unlimited Ammo
Hold shift and hit tilde (~) to bring down the cheat menu. Enter "Impulse 911" to get unlimited ammo.
Walk Through Walls
While playing, press SHIFT and TILDE (~) to enter the menu, then type "wraith" to walk through walls. Enter it again to disable this code.