I second the Niterider series. I had their front Lumina, when I realized I needed a brighter light to see all the potholes on my winter evening commutes. And got this exact rear one when I needed to upgrade my back light.

I love that it is rechargeable, and bright enough to be seen in the deceptively lit suburban light-pollution of my commute. (And they are pretty easy to find at sports/bike shops)

And the grips are good enough to hold and readjust, but can still take them off if you lock up your bike for a few hours.