Various cheats
     Press ~ to enter the console then type:

Cheat code:             Effect:
Fly                     Flymode On/Off
NoClip                  No Clipping On/Off
Impulse 11              God Mode
Impulse 43              First of the five impulses that cause death
Impulse 69              Second of the five impulses that cause death
Impulse 137             End Credit Music
Impulse 151             Third of the five impulses that cause death
Impulse 666             This is the impulse that crashes the game
Map n                   Warp (where "n" is the level name)