There's the crappy flashlight pistol mod, and then there's the ULTIMATE light mod.... DUCT TAPE.
(they may or may not be the same thing)
I haven't tried it yet, but from what I read, its supposed to be very cool, and the guys who made it have really tried to keep the game feeling the same, just with a little extra light.
You can find it here:
http://ducttape.glenmurphy.com/
Under the crazy presumption that a roll of duct tape has to exist somewhere on the Mars facility, the Duct Tape mod sticks flashlights to your machinegun and shotgun. In order to preserve the atmosphere, these new lights are much narrower (and a little brighter) than the standard flashlight, and are only available on the basic weapons. The pistol is not equipped with a flashlight, so as not to spoil the early sections of the game.