Noted for future reference:
Trackmate is an open source initiative to create an inexpensive, do-it-yourself tangible tracking system. The Trackmate Tracker allows any computer to recognize tagged objects and their corresponding position, rotation, and color information when placed on a surface.
Trackmate uses a small, specially designed circular barcode that stores information which can be easily decoded by the Trackmate Tracker. The tag measures less than 1″x1″ square and contains a six byte unique ID (over 280 trillion unique IDs are possible).
(Via Make Blog.)