Cheat: HOLLOW MAN: Makes you invisible. However, an important advice: do NOT enter a building even with this cheat on - enemies inside will perceive (and gun at) you in any case.
Cheat: POWERMAN: Makes guns more powerful. I personally have not noticed anything with the handguns, but the Gatling guns get an unlimited supply of ammunition.
Cheat: JACKAL: Is supposed to provide unlimited ammunition. The only change I have noticed is that Doc McCoy never runs out of sniper bullets when the cheat is active.