Nite rider makes a good, affordable helmet mount light.
http://www.performancebike.com/shop/...slisearch=true
This one is currently on sale for $89, and is a very good light. One of my buddies uses it several times a week. Rechargable battery lasts 2-3 hours at a time.
For $200-300 you can get a very good light from NIterider that is quite a bit brighter and will last 2-3 times as long between charges.