If you are in the market for a taillight that will give you night and day visibility, Hotshot by CygoLite is your answer.
This is the best rear light we carry. It's USB rechargeable, so no more pesky batteries to replace; just charge it up with your computer and go. It gets dark pretty early, here in Seattle; this little Hotshot features a full 2-watts of LEDs, that's roughly 100 Lumens of brightness. It boasts a total of 5 variable flash settings, giving you a battery life of up to 500 hours. Mounts easily to your rack or seatpost.
USB rechargeable lights are the future, and the future is nigh!
At $39.95, you can't afford not to be seen!