a range of devices are capable of observing a person's pupil to determine the direction of their gaze.
Eye-tracking can be used for input, by directly controlling a pointer on the screen, or even for communicating the gaze direction to a co-worker looking at the same document (much like a telepointer).
Eye-tracking can also be used to study how user interfaces are being used and may give insight into screen layout issues and how users are performing their tasks.
