Full Health and Ammo:
At any point while playing, press M, I, L at the same time.

Debug Mode:
In the DOS promt type "sod" (or "spear") followed by a space and a minus sign and type debugmode.

Eg: C:\SPEAR>spear -debugmode

Then when you are in the game hold Alt, Shift, and Backspace at the same time. A message will appear.....

And then use the following:

TAB + G - God mode
TAB + B - Border colour
TAB + I - Items and lots of points
TAB + H - Hit yourself
TAB + N - No Clipping
TAB + E - End Level

Thanks to Revolution readers Brandon, Tom Noble, and H. Zorzano!