EDTracker is a DIY project, so is a bit more involved than the rest.
The cheapest and fastest way to get some headtracking is facetracknoir : all you need is a webcam. Facetracknoir tries to track your face orientation in the webcam picture. If you want to up that you could either 1) build yourself a three point led clip or 2) buy one (just search ebay for trackir. it should cost around 30 euros for an official TrackIR replacement clip and much less for inofficial ones). This works best with highspeed cameras without IR-filters. The PS-Eye is such a webcam (up to 120fps, costs 9€ on amazon), but you''ll need drivers which, understandably as they were reverse engineered, cost a couple of euros (
https://codelaboratories.com/products/eye/driver/). The facetracknoir can easily use the 3 leds to triangulate your face orientation. This is, as far as I've seen, much more reliable and less CPU-draining than real face recognition. It is sadly a bit reliant on good lighting condition as sunlight being reflected from surfaces can create random 'hotspots'.
If you're at the point of shelling out for IR Clips and possibly a better webcam you might as well grab a prebuilt EDTracker (
http://www.edtracker.org.uk/index.php/shop) I heard only good things about it (but I've been using free face tracking for ages so I already had all I needed for E : D, so never tried EDTracker myself)
If you're into "I only use 'professional' commercial products because the more expensive something is the better it MUST be" and money is not an issue for you then you can go and buy an official TrackIR device. those are particularly expensive (Amazon.de lists the "complete TrackIR incl. clip" for 180-200€). They work the same way as FacetracknoIR with a Led clip. It should be noted that NaturalPoint, the manufacturer of TrackIR tried to patent "triangulation of 3 points" and to shut down free alternatives using those basic mathematic principles, so whether you want to support them might be an issue.
Whatever you chose, a good working face tracker setup is absolutely awesome in simulators and it's phenomenal in E

. In my experience, it doesn't work as well in infantry simulators like Arma2 or 3 though as you are also moving your
body in a different direction, as opposed to sitting in a cockpit and just looking around.