For tail lights these are great!

http://www.teamestrogen.com/products.asp?pID=17576

I have a Light and Motion Vega that I'm very happy with. It's got about a four hour run time on the lowest setting, two hours on the highest. It's not very heavy. It did interfere with my computer until I mounted them as far from each other as possible. Then I broke the computer speed sensor. It does not interfere with my Polar.

V.