The Leap Motion is a movement tracking computer interface which is small and relatively inexpensive.
The Leap Motion Controller tracks your hands at up to 200 frames per second using infrared cameras. This is then converted into input.
There are lots of apps developed to use with it, including some music-making ones.
We tested it when it first came out in this video:
Since that video was made, Kieran Plissonneau has made enormous progress with his Sonic Leap project.
This has much improved its accessibility and it now responds to pointers and a range of hand physicalities.