The Blackburn Mars 3.0 rear bike light is excellent.

No other rear light is as bright at this price point - it actually does look like the rear light of a car on the road at night, so you are guaranteed to be seen. (Though for belt and braces, I also use a tiny flashing LED light by Topeak as well).

It's a small, lightweight light and it comes with a variety of fixings to ensure that it fits to any bike or rack. Burn time is pretty good with a set of high power rechargeable batteries. It's a bit fiddly to change the batteries, but that shouldn't put you off. If you ride on the road in darkness and value your safety, buy this light. It's that simple.

A good effiicient light, with a choice of frame mounting (if it will fit somewhere it is visible) or a belt clip, but changing the batteries is best done at home.