I have the Serfas TL-One. I really like it. It was about $25 at my LBS. Super bright with 1 steady and 3 flash modes. Another thing I really like about it is the ease at which you can put in on and take it off your seatpost. The light has a rubber strap mount that is super-easy to use, and, better yet, doesn't require a semi-permanent mount on your bike. This means it's only there when you need it.
My riding buddy was so impressed with the Serfas when he saw mine, that he went out and replaced his tail light with the TL-One also.
I couldn't find a website, showing the mount, but here's a good view of the light.
Serfas TL-One Taillight from BikeBling.com
edit: I just looked at the BikeBling site again. If you click "more images" they show a mount. This mount differs from the one that came with my light. The one with my light is a rubber strap with different notches to accommodate different post diameters instead of the plastic quick release shown in the picture.