A click in the night.
You don't have to be afraid of the dark if you have Blackburn's Click USB Rear Light with you. This LED light shines 20 lumens to make sure you're visible to overtaking traffic, and built-in side illumination makes you noticeable to cross traffic as well. Its rubber mounting strap is compatible with seatposts and frames 15 mm - 35 mm in diameter, and the integrated Li-Po battery is USB rechargeable in 3 hours using the included cable. Remaining battery life is shown with a built-in indicator.
Runs for 1.5 hours on constant and 3 hours on blinking mode. IP65 waterproof rating. Claimed weight is 20 g. Available in multiple colors.