I'm pretty sure you've already seen some demos of Microsoft Surface (if not, look here). Pretty amazing and sci-fi, eh? Also sci-fi is the ReacTable which allows building sound synthetizers by just moving cubes around on a surface.
The hardware for these things for a cheap personal setup is actually quite simple, and we now have lots of free software to power it and do amazing things. So during this rainy week-end afternoon, I've built my own mulit-touch table! I won't post pictures as it's really ugly for now ;-)
All it takes is a big cardboard box, a piece of Plexiglas, some tracing paper to be taped on it, a webcam, and there you go. I started exactly like this guy demonstrates. It's that easy! In my setup I've added a diffuse light inside the box so that touch detection is more accurate.
Now I wanted to do some ReacTable-ish stuff, using real objects and not only my fingers. The Reactable people have open sourced their fiducial and finger tracking software in the Reactivision project, and there's a recent project to build a Reactable clone.
Yay! Now I can make music (more exactly sounds at the moment) with my tangible multi-touch table!
There's a big piece missing though: I move fingers and stuff on the table, and see the visual feeback on my laptop's screen. I need a projector to finally have the real thing, but decent projectors are still expensive. Hmm... I'm sure the fun is certainly worth the price!