In reply to Adam Coles:
I use Petzl Myo 3, which I think is excellent. Most headtorches will fit any helmet which has head torch clips.
Only other recommendation is that if you want a headtorch for serious use you should get one with both LEDs and a halogen bulb. LEDs are great as they don't use much power, and are good for inside the tent, walking back from the pub etc. However if its foggy or really heavy rain LEDs don't penetrate at all, you just get a glow around your head.