I'll give a +1 to those who recommend the Planet Bike Superflash for a tail light. Seriously. I run 2 of them, along with a Planet Bike Binky7.
It works. Big time.
Look for the other thread that's going on here where we al talk about how wonderful PB's customer service is.

Its a great outfit, and they make great products. Get their light. You won't regret it.