Start the game with parameter "level1diagnostic" Next use [Ctrl]+[Left Shift]+[Tab] to activate the cheatmode.

Now you can use []+

[W] : Level Warp
[G] : God Mode on/off
[N] : No clipping
[S] : Freeze game
[B] : Change border
[C] : Pauses game and displays useful info
[F] : Display your compass angle
[I] : More score, more ammo, more charges, and next weapon
[M] : Memory check
[R] : Red skull in front of character
[T] : Shows info on current level's transporters [CD ver only]

Or:

Press [W]+[A]+[X] for all weapons/ammo, keys, a few mines, full map, and full health.

Or:

Use commandline parameter "president"