Press ~ to enter the console then type:

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)

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