CDM points out this interesting demo of a tangible tracking system using Trackmate software

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 sends all object data via LusidOSC (a protocol layer for unique spatial input devices), allowing any LusidOSC-based application to work with the system.

Seems like a simpler alternative to the Reactable software – and tutorials are already available for building your own interface surface. Looks like fun … might just have to give this one a try myself.