Press ~ during game play to display the console window, then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
All weapons - gimmiegimmiegimmie
Cycle five chase camera angles - chasecam
Cycle four player classes - cycleplayerclass
Extra armor and health - nextarmor
Fly mode - spectator
God mode - invuln
Health debug mode - debug1
Less armor - prevarmor
Spawn indicated item - give
List items for give code - give
List console commands - listcommands or set
Next monster - nextmonster
No clipping - noclip
Previous monster - prevmonster
Restart level - restartlevel
Toggle monster targeting - notarget
Weapon debug mode - debug2
Unknown - cameralock