So unless you are living under a rock, you have probably heard about the new VR-google craze soon to hit every hard-core gamers cave (e.g. Occulus Rift). We talked about the FOSS engine getting Occulus support before, and now that id software promised to release the Doom3 BFG Edition source code too, it looks like VR in FOSS games will become quite common soon.
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.