Microsoft uses the Doppler effect to sense gestures (video)

Gestures are becoming an increasingly popular means of interacting with computers. However, it is still relatively costly to deploy robust gesture-recognition sensors in existing mobile platforms. SoundWave is a real-time sensing technique that leverages a speaker and a microphone to robustly sense in-air gestures and motion around a device. It is capable of detecting a variety of gestures, and can directly control existing applications without requiring a user to wear any special sensors.