+1 for the Hotshot. Completely visible in day time, USB rechargable, small size, and the rack mount means you can adjust it for optimum rear-facing visibility (as it has a somewhat narrow focus).

I use it in conjunction with a PDW Red Planet 5.0--the Cygolite I run on the rack as my main light and the PDW is on my seat bag because it has very good sideways visibility.