I have one of those tiny iR/white Princeton lights. The cheaper lightweight one that uses the small coin battery. It'll fit on a helmet rail, picatinny rail, molle loop or regular helmet mount, and doesn't use much space. Not super powerful, but I have that on my rifle. It allows other things to be mounted too.
TNVC has all kinds of mounts and such, and there are all kinds of lights depending on how big, heavy or powerful you want it. Surefire makes some nice stuff, including a small single batt. IR only light. The Vampire looks nice and has white light too if you want something big.