Head tracking does just that; it tracks the position and movement of your head and emulates it within the game. The result is you can freely look around your environment – it’s essentially a poor man’s VR. And, I couldn’t love without it.
The cheapest option is FaceTrackNoIR, a completely free program that uses your webcam to track the movement of your face. It isn’t perfect but it is a reasonable introduction to the concept before you drop any money.
At the most expensive end of the scale is TrackIR, which can cost up to $169.95. It comes with a clip to fit onto a pair of headphones with 3 LED lights and an infrared webcam-style device to track those LED lights.
I personally purchased the TrackHat Clip with the modified webcam and it has been superb but I have heard equally good things about the DelanClip.