A HeadTracker is often used in simulation games to look around without the need to move your hands away from your wheel, joystick, keyboard, mouse or whatever you are using. But HeadTracker are quite expansive. FaceTrackNoIR is a cheap alternative and uses your normal webcam to track your headmovement.
How it works
Most HeadTrackers use infrared LEDs mounted to your Head in some way to triangulate your movements.
FaceTrackNoIR, as the name says, does this differnetly. It uses your webcam image to triagulate between your eyes, nose and mouth. That way you don't need any additional equipment besides a webcam and a bright room.
First you need to buy the software. It costs about 3€ and you pay with PayPal.
Shortly after you will recieve the exe via your EMail.
Once you installed it, you only need to choose you webcam as an input. After that you should see a preview of the tracking in the window on the right.
Most games detect the FaceTracker as a mouse input and need no further setup.
But you might want to tweak some of the tracking-settings in the FaceTrack Software.
There are many curves that you can tweak to your liking.
You can also disable the tracking on certain axis. For example in a racing game you might only want to enable the x-axis since you dont need to look up and down.
You can also store your settings in different profiles for different games.
If you like to play simulations you should try FaceTrackNoIR. It is not expansive at all and will lead to a more immersive experience over all.