Someone else has already mentioned this light, but I too recommend a phaart bleep from Planet X.
One of the best value things I've ever bought - I think I got if for £1.50, but at the moment they are selling them for £3.99. Not exactly expensive.
It's not rechargeable, but I think I get about 1000 miles between changing the two AAA batteries, and I always put the light on, even when riding in daylight.
Just zip tie it to the seatpost (throw the bracket away) and forget about it. Can't see how this product could be improved.
It's very bright too - the only thing I can't vouch for is how it would fare on a bike without mudguards, as the button to turn it on would be in the full force of the water thrown off the tyre. With mudguards, I've had no problems with water getting in whatsoever.