The AirPiano is gesture sensing MIDI controller. It consists of an Arduino board and IR sensors, the prototype instrument provides the user 3 layers each of which contains 8 individual "virtual switch". When a switch is triggered a MIDI note message is passed and a LED under the switch is activated to provide the user with visual feedback to aid in the learning of the instrument.
The developer has posted 2 youtube videos of the AirPiano in use:
The first demonstrates its use a controller for Ableton Live.
The second demonstrates its use a more traditional melodic MIDI controller.
Whilst it looks like a fun thing to play around with I'm not convinced by it really. I don't really see the advantage over a 'normal' MIDI controller which gives you tactile feedback. The developer suggests that it could be used to make a more interesting performance which is true, but a 'normal' MIDI controller can still be played as an instrument. I think that there could be potential to do some interesting things with this controller given more time spent experimenting with it.
More info can be found at airpiano.de