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Open the command window by pressing the key between ; and #.

Code	Effect
god	invincible
allammo	999 ammo
loaded	all guns/weapons and 999 ammo
playersonly	freeze (type again to undo)
killbots	kill bots( (may be irreversible during that map)
ghost	ghost i think
fly	no need to jump you can fly!!!
walk	changes you back from fly
teleport	teleport to crosshairs