Audio Location

Audio Location, developed by James Scott at Intel Research Cambridge, is a low-cost location sensing mechanism based on the use of off-the shelf microphones attached to cheap PCI sound cards in a standard PC.

The system doesn't require the user to wear any tag, it can detect human-made sounds such as the clicking of fingers or clapping to accuracies of around 15cm for a 3D location.
Possible use of the system include any application which might otherwise involve a real button interface. Finger-clicks on virtual 'buttons' in 3D space could control lights, music players, etc. On a computer desk, audio location might be used to quick-start or control often-used computer applications. PDF of presentation.

Related: echolocation which scientists would like to explore in order to help blind people detect objects with sound.