This thing is pretty damn sweet.
You use these blocks called tangibles on what they call a "tangible user interface" to create different sounds and effects. Apparently Bjork has used it a few times. They've even released an app version for your iDevices.
DJ using the reactable for some trance music.
There are also a few exhibitions around the country such as one at the Museum of Science and Industry in Chicago, or Discovery World in Milwaukee.
Watching the videos is kind of confusing to me. I understand the basic concept of how the thing works and how each tangible interfaces with the board. Though they do have a tutorial on how it works on their website.
And don't forget you can own one today for only €9.700 or $13,679.91!