Enter the cheats in the console window (Activated by pressing ~):

god = God Mode
fly = Fly Mode
walk = Walk Mode
ghost = Walk Through Walls
allammo = All Guns, Ammo
slomo # = Sets Gamespeed to # (1.0 is normal) killpawns = Kills ALl Monsters
playersonly = Freezes Time, Type Again to UnFreeze
invisible 1 = Makes You Invisible
invisible 0 = Makes You Visible Again
behindview 1 = View Character from Behind
behindview 0 = Normal View
say [message] = Broadcast [message] to Other Players
summon [class] = Spawns Actor of the Specified [class]
killall [class] = Kills All Actors of a Certain [class]
switchlevel [new level] = Switch to a New Level switchcooplevel [new level] = Server Switches to New Level w/ CoOp Style Weapon Carrying