Made for the use at a bike: The LED Taillight IXXI by Busch & Müller. It is great because of its simplicity and practical extras: This is why it comes with a rechargeable battery that can be quickly recharged via a microUSB. Moreover, the whole lighting unit can be simply removed from the case and put into the pocket. It also has a small capacity indicator telling you when to recharge. The LED Taillight IXXI gets you safely through road traffic.
Note: Does comply with German road traffic regulations. ;"