I've this light:

http://dealextreme.com/details.dx/sku.14909 ($18, free ship)

If you're riding in pitch black conditions at speed (20+mph) its probably not enough. Also, it drains batteries, especially in the cold - I was riding when it was 20 degrees, and by the time I got back (after only ~40 min of runtime on 2500mah rechargeables) it was really dim. I need to wrap it in insulation or something.

btw, anyone find a solar-powered taillight an odd idea? It works for lawn lights or such that are in the sun all day, but I usually take my lights off and stuff them in a bag.