Leave it to the Danes - though Reelight requires careful installation, you are battery free with these 'daylight running lights' that flash when the wheels rotate. They work by small magnets attached to your bike spokes, which create electric current enough to power the lights' LEDs each time the magnets pass the lights. As long as the wheel rotates, you get flashing red or white. The Reelight is a great step beyond static reflectors.
Photo of a red Reelight mounted on the bike back via 36widgets @ flickr.