Check out OptiTrack at NaturalPoint

http://www.naturalpoint.com/optitrack/

OptiTrack is your complete optical tracking solution. From the camera to the software interface, it is everything you need to develop your own tracking applications! OptiTrack offers unique flexibility with its simple USB camera interface and powerful software API, a sample tracking application is included to have your system running right out of the box. Whether you need a single camera system, or want to have a room full, OptiTrack is the motion capture solution you need.