Press ~ to bring down the console. Then type in the following cheats and press Enter
to activate: 

allammo        All ter'angreals w/infinite ammo
behindview #   Change view (replace # with 1 for Third-person, 0 for First-person)
GOD            Invincible
fly            Fly mode
ghost          No clipping
walk           Return from GHOST or FLY mode
killspawns     Kills all monsters
killall (class)Kill actors fromone class
playersonly    Freeze time (repeat to unfreeze)
invisible #    Invisible (1 or 0 for visible) 
slomo #        change gamespeed (1.0 is normal)