I think a 1 watt LED headlight is the best value now. It runs for long time and the light is rather powerful for the lighted streets. Performance sells one that has rechargable batteries.
Planet Bike Super Spot 1 Watt LED Headlight for $29.
If you want more light, you can buy 2. It'll be 8 batteries for 30 hours.
http://www.nashbar.com/profile.cfm?sku=11266