Cheats
Press the ~ (tilde) key to bring down the console, then type these codes:

Bigfinale	Play ending sequence
Fly	Fly mode
impulse 11	God mode
impulse 42	Technically an Easter egg Ð try it and see
Noclip	No clipping mode
ode to jack	Kill all enemies on level
impulse 43	Suicide
impulse 69	Another suicide