Phuture.com

Ran across this project when researching some video recognition systems, installed the software, connected a camera..and amazingly, it all worked. There are more detailed videos and instructons online, so search around a bit if you are interested in exploring this. (videos inside)

reacTIVision 1.3:reacTIVision is an open source, cross-platform computer vision framework for the fast and robust tracking of fiducial markers attached onto physical objects, as well as for multi-touch finger tracking.

Here is a demo with the group's proprietary visualization software.

Here's a video demonstration using a webcam and ableton live, more on youtube:

In my brief experiments, I was able to use a midi router (like midi yoke) to recognize and send the data, and it's really not too difficult to setup.