This thing is by far one of the COOLEST gadgets I’ve ever seen. Watch the videos – it’s hard to explain what it does without seeing it in action. The summary on their page is:
The reactable, is a state-of-the-art multi-user electro-acoustic music instrument with a tabletop tangible user interface. Several simultaneous performers share complete control over the instrument by moving physical artefacts on the table surface and constructing different audio topologies in a kind of tangible modular synthesizer or graspable flow-controlled programming language.
The wonder of it all is the entire thing is opensource and downloadable. Build your own!
Thanks to HackADay.