However, hidden in the (flight-)sim genre another quite nice system has been developed, using only a (sufficient frames per second) webcam:
The video is shot with FlightGear, everyones favorite open-source flight-sim. More details how to get it running with FlightGear can be found here, the system itself is not FlightGear specific though.
The source-code can be found here to be adapted to to your game (any 3D game that doesn't require too fast head-movement is basically suitable). The face-tracking is based on OpenCV, which will take some juice from your idling quad-core CPUs ;)
Less resource demanding are infrared LED tracker version, which do not need to follow a face and also work rather nicely in a dark room. For those, some propitiatory solutions have been available for some time, but you can also find Linux compatible open-source code for such a system here (instructions for FlightGear here).
Personally I was always to lazy to build myself a proper 3 dot LED cap, so I think the face tracking solutions are more convenient. If you are into non-FOSS games on the Windows platform, I can thus also recommend the partial open-source FaceTrackNoIR software, which supports quite a few really nice flight-sims, racing games and even FPS.