Run the game with the '-devuser' command line parameter and then during gameplay press the '~' (tilde) key to bring up the cheat console window. Then type in the following passwords (without the quotes) to activate the desired effect in the game.

Unlock Invulnerability:
Enter the password 'God'

Unlock Maximum Ammo:
Enter the password 'Ammo'

Player/Game Speed Accelerated by # (replace # with number, default is 1):
Enter the password 'Speed #'

Bug Entities Die instantly when Crosshair is placed over them:
Enter the password 'Evileye'

Unlock Combination of the God, Evileye and Speed 3 Cheat:
Enter the password 'Banzai'

Change Gun Model:
Enter the password 'Gun'

Unlock HUD:
Enter the password 'Hud'

List ALL Codes:
Enter the password 'Help'