I run 2 Cygolite Hotshots, IMHO the best bang-for-the-buck light you can buy. Many people say the Hotshot is very directional, and it is, but from 50 yards to 1/4 mile and even farther away, these lights *WILL* be seen by approaching motorists behind you. I have one mounted down low on the left seat stay, and the other on the rear of my Topeak rack (with the separately-purchased Hotshot rack mount).

The Hotshot recently had some improvements made to the included seat post mount, and also now include a dedicated seat stay mount. It uses 2 separate screws, one for tightening the clamp around the stay or seat post, and the other for securing the angle of the mount bracket. It's a really nice improvement over the original version (I have one of each). Also when charging, the old version used a separate internal flashing LED to indicate charging. The new one uses the main 2-watt LED in extremely low power mode to indicate charging. The new version also indicates when battery power is low by strobing the LED for a couple seconds at very low power, either when you turn the light on or off.