From the Reactable website:

The reactable is a collaborative electronic music instrument with a tabletop tangible multi-touch interface. Several simultaneous performers share complete control over the instrument by moving and rotating physical objects on a luminous round table surface. By moving and relating these objects, representing components of a classic modular synthesizer, users can create complex and dynamic sonic topologies, with generators, filters and modulators, in a kind of tangible modular synthesizer or graspable flow-controlled programming language...

The reactable hardware is based on a translucent, round multi-touch surface. A camera situated beneath the table, continuously analyzes the surface, tracking the player's finger tips and the nature, position and orientation of physical objects that are distributed on its surface. These objects represent the components of a classic modular synthesizer, the players interact by moving these objects, changing their distance, orientation and the relation to each other. These actions directly control the topological structure and parameters of the sound synthesizer. A projector, also from underneath the table, draws dynamic animations on its surface, providing a visual feedback of the state, the activity and the main characteristics of the sounds produced by the audio synthesizer.

Bjork is collaborating with the Reactable team onstage on her current Volta tour. Here are some more demos of this wonderful creation.