Ctrl F10: god mode
F10: all weapons
F: displays framework
CTRL-V: displays game stats
F7: displays grid over level
ALT-F7: displays another grid
CTRL-F7: displays yet another grid
Jassica16: Super Powerups!
H : Enable Item Mover

In addition to the in-game stuff, here's some more cheats that are entered on to the end of the EXE line you use to start the game. Suffixes, if you will.

 
-warp ??                 = Level warp, where ?? is the number of the required level
-skill ??                = Change default skill level , where ?? is a number 1 to 99
-asylum                  = Displays message 'Enabling ENHANCED mode. (Not!)' and that's it.
-demo                    = Will continuously loop intro.
 

Oh, and one more thing. Amazingly, the old No Remorse ammo bug still works. Just drop your weapon then pick it up again to find that it's been refilled. Mad.

Thanks to Revolution readers Yay Yay, M. Colasurdo, Jesse Adams, and Satan!