Press ~ to enter the console then type:

FlyFlymode On/Off
NoClipNo Clipping On/Off
Impulse 11God Mode
Impulse 43First of the five impulses that cause death
Impulse 69Second of the five impulses that cause death
Impulse 137End Credit Music
Impulse 151Third of the five impulses that cause death
Impulse 666This is the impulse that crashes the game
Map nWarp (where "n" is the level name)

Thanks to Revolution readers Matt “Wings” Langis and David Zahler!