I build musical and visual instruments with a variety of 3D controllers, including the Leap Motion device. Tim wrote an open-source app in Python which implements a musical instrument using the Leap Motion. You use one hand on a MIDI controller to control the notes of a scale, and you use the other hand with the Leap Motion to play freely within that scale. Each finger plays a different note, and all three dimensions are used – x is pitch, y is quantization, and z is velocity.