Cheat Mode:

Press ~ (tilde) to bring up the console window and use the codes below:

Cheat      Mode

xfly        - Fly Mode
xghost      - No Clipping
xwalk       - No Clipping/Fly Mode off
xgod        - God Mode
allammo     - Unlimited ammo
killpawns   - Kill All Enemies
setjumpz #  - Set Jump Height (500 = Normal)
setspeed #  - Set Speed (2 = Normal)
summon #    - Summon Item 

Thanks to Revolution reader Fabkill!