Ferromagnetic Input Device (with ball bearings!)
An interesting project from Microsoft Research that uses an array of magnetic sensors to allow the detection of “any ferrous object placed on or near its surface”. The BeatBearing is referenced (in the paper below) as a possible future application, which would allow the sensing of object above the surface, and other properties such as rotation of the ball bearings. Both of these features would open up a whole lot of new possibilities!
Video link: YouTube