This appeared in a different thread already, but I think here might be the right place to put this announcement:

reacTIVision is an open source, cross-platform computer vision framework for the fast and robust tracking of fiducial markers in a real-time video stream. It was mainly designed for the rapid development of table-based tangible user interfaces.

We also provide a library and a demo sketch for processing which you can use for debuging purposes or as a starting point for your own sketches. The demo receives the TUIO messages from the reacTIVision application and displays the objects' ID, position and angle on the screen. From a developer's point of view you will need to implement  the addTuiObject, removeTuiObject, updateTuiObject and refresh methods which will be called when these events occur.

The reacTIVision engine, the example sketch for processing and other environments are available here:
http://www.iua.upf.es/mtg/reacTable/?software

cheers,
Martin Kaltenbrunner
Music Technology Group
Barcelona, Spain