Press TAB while playing and then type one of these
codes (If TAB doesn't work, type ~ instead of TAB):

allammo - All Ammo
behindview 1 - External View
behindview 0 - Normal View
flush - Fix bad textures on walls or creatures
fly - Fly Mode
ghost - No Clipping Mode
god - God Mode
hideactors - Hide all monsters\weapons\...
showactor - Show all monsters\weapons\...
invisible0 - Invisibility off
invisible1 - Invisibility on
killall (monster) - Enter the name of the monsters you want to kill
killpawns - All monsters on the current level are dead
open (mapname) - Levelselect
playersonly - Timer freezes
slomo (number) - Change the speed of the game (1.0 is normal)
summon (item) - Summon weapon, item or monster
suicide - Suicide
walk - Walk Mode (use it after pressing ghost or fly)