Been riding for a while with a Niterider Lumina Flare on my helmet:
Large pizza; tiny bike - challenge accepted by 1nterceptor, on Flickr
It's rated for 650 lumens; but there are more powerful models that will fit their helmet mount.
Lumina 750, 900, 100, etc. I like the mount very stable and has a ratcheting action to tilt the
light up or down a bit. I thought I lost my light during my last move. Got a Planetbike Touch800.
Also a nice light; though not really designed for helmet mounting. Will come up with a review/comparo
in a little while if you can wait. If you need one right away; can't go wrong with the Lumina sries
and an optional helmet strap.