There are lots of tiny emergency LED lights around these days, but we particularly like this one from Electron Backupz.
The elastic tail wraps around your seatpost for a secure fit, and although they're tiny, the five perimeter LEDs are surprisingly visible. There are two flashing modes together with one constant mode, and the light is powered by the supplied CR2032 batteries that last for months. We used this all last winter without any problems, and, at 20g, it weighs very little. It even works as a key ring.