FreeTrack is a webcam-based optical motion tracking application

FreeTrack is a free optical motion tracking application for Microsoft Windows, released under the GNU General Public License. Its main function is inexpensive head tracking in computer games and simulations but can also be used for general computer accessibility, in particular hands-free computing. Tracking is sensitive enough that only small head movements are required so that the user’s eyes never leave the screen.
(Link: FreeTrack is a webcam-based optical motion tracking application)