Type them in the " ` " console:

iwillsurvive - God mode
raindropskeepfallinonmyhead - Allow call for artillery; can be re-entered for more
takeittothelimit - Unlimited ammo
hitmewithyourbestshot - Computer rocks ! ! !
closetoyou - Enemy cant see you
gamma # - Set gamma to #
turbo - Run faster

Level Select
While playing, press tilde (~) to call up the console window, then type "letmego" and press ENTER.

Reduced Sky Details
While playing, press tilde (~) to call up the console window, then type "sky" and press ENTER. This speeds up the game and makes the world appear brighter.