I love head tracking options in games but FaceTrackNoIR is the best solution hands down to this implementation. Track IR is very agressive in demanding no other similar devices can be used in software if their product is supported. FaceTrackNoIR is free for all and doesn't require anything more than a decent webcam. It would have the broadest usage as it doesn't require dedicated expensive hardware to use it.
facetracknoir.sourceforge.net/
I have to disagree with you about FaceTrackNoIR being the
best solution, certainly from the technical side of things. For me, FaceTrackNoIR just didn't work well enough, and was far too inconsistent and unreliable. It is however, highly dependent on the quality of the webcam you use - as you said - so your mileage will vary greatly because of that. It's also very sensitive to ambient light and so on. It also helps to be handy with a soldering iron, as a lot of the webcams available have an IR filter that you'll need to disable before they'll work properly, and like the TrackIR, you'll get best results using LED's instead of passive tracking via the camera, and you'll need to put that together yourself. Conversely, TrackIR just works, straight out of the box, and works extremely well. You also often end up jumping through hoops to get the open source software to work well, and end up trawling the web looking for people who've written plugins for the various titles that actually work well, rather than the standard ones that I, and many others, couldn't get good results from.
I can't speak to them demanding exclusivity in terms of games that support head-tracking - the DCS titles certainly support TrackIR AND the open source solutions, and many of the titles I see on their site are titles I remember seeing TrackIR support for. I do however understand their fees are very high if you want to license their product, or part of it, and the device itself certainly isn't cheap at all.
Don't get me wrong, I'm not trying to discourage people from looking at alternatives to TrackIR, and I can attest that FaceTrackNoIR is certainly a good one to look into. But to get it to work anywhere nearly as well as TrackIR does, you need to fiddle and fuss with it a lot, and you may not succeed at the end because of the webcam you're using, or because of bad software support. If you try FaceTrackNoIR, or something similar - and I encourage you to - and get good results, fantastic. Just be prepared to do a lot of it yourself, to have to tinker and fiddle, and to chase the kids out of the room from time to time so you can swear properly.
But a lot of people are going to end up doing what I did - try to do it themselves, see how brilliant the concept is, but get frustrated with how badly they get it to work, and end up splashing out the money for something that you just plug in, and play with.