The astrotrac work as advertised, and have the distinct advantage of being extremely portable (everything packs into the pier and into your backpack).
http://www.astrotrac.com/Default.aspx?p=pier
The Pentax K-r and K5 DSLRs have tracking available in the camera itself when connected to the O-GPS unit.
You calibrate the camera and can shoot on a standard tripod and the sensor will track, allowing you long exposures without a special mount. It works really well.
http://www.pentax.jp/english/product...trotracer.html
There are all the barn door style trackers but I have yet to try one.
Most of the other commercial camera track mounts I have seen are rather large or clunky compared to the astrotrac.
http://www.astrotrac.com/