Forgive me if this has been shared before, but I found it quite interesting and I just stumbled upon it via a Twitter friend.

It is a musical instrument driven by a Mac computer which uses a projector to provide a table-top touch interface -- similar to the way Microsoft Surface works. Watch some of the tutorial videos, they are pretty neat! If only the price tag wasn't so harsh.

Edit: It appears there's a Reactable app for iPhone and iPad as well, if you can't afford the Reactable Live! instrument.