I have to second the Deal Extreme lights. I have this exact setup for about $60.

Light: http://www.dealextreme.com/details.dx/sku.16092

Battery: http://www.dealextreme.com/details.dx/sku.5790

Charger: http://www.dealextreme.com/details.dx/sku.6105

Mount: http://www.dealextreme.com/details.dx/sku.8274

I can swap the light between bikes as I have a mount on each of my two bikes. The light has 5 settings, the fifth is insanely bright but lasts for about an hour, other bikers and motorists complain at this setting. The next highest setting is about right and lasts for about 3 hours. I always carry a spare battery with me but have never needed it. Batteries charge pretty quickly. I also supplement with a Planet Bike Blaze on my helmet which I sometimes run on flash mode as a "to be seen" light. As an aside I have used the light for other things like spotting leaks in my attic, peering behind furnaces, under beds, etc. It is an extremely useful light in that regard and blows away my Mag Lite.