Activate the following cheats by entering the corresponding codes at the console menu:

CHEAT FUNCTION - CHEAT CODE
All spells - allammo
Breathe underwater - amphibious
Disable invisibility - invisible 0
Enable invisibility - invisible 1
Fly mode - fly
God mode - god
Kill all monsters - killpawns
Kill indicated classes - killall <class>
Level select - switchlevel <level>
All levels completed - completelovol
No clipping mode - ghost
Normal view - behindview 0
Select new coop level - switchcooplevel <level>
Sends message to other players - say <message>
Set game speed - slomo <0.0-1.0>
Summon indicated class - summon <class>
Third person view - behindview 1
Toggle time on and off - playersonly
Walk mode - walk