A Serfas Thunderbolt is a great tail light. The only knock I have against it is that it only lasts about 4 hours so if I'm out for a long ride it might die on me. On the plus side though, it is really easy to mount just about anywhere, has good side visibility, is insanely bright straight on, and is USB rechargeable so if you use it while commuting you can charge it during your work day.