This neat little light from
It uses seven LEDs to provide the light, and is powered by two supplied AAA batteries. The water resistant case is robust, and as well as having a built in mount, comes with a seatpost bracket should you wish to mount on your bike.
Five of the LEDs face straight back at the traffic, while two of them flash through orange casing to act as sidelights. You can pick between flashing, constant and a scrolling effect, which draws more than enough attention to you.