headtrackr is a javascript library for real-time face tracking and head tracking, tracking the position of a users head in relation to the computer screen, via a web camera and the webRTC/getUserMedia standard.
For a demonstration see this video or try out some of the examples with a laptop that has a camera and a browser that has camera webRTC/getUserMedia support (for instance Opera 12). For an overview of browsers supporting the getUserMedia standard see http://caniuse.com/stream.