Some of you may remember it, as it was publicly available for the famous Commodore C-64 home computer: the Light-Pen. A pen with a phototransistor tip that worked in conjunction with cathode ray tube displays. The Light-Pen detects brightness changes of the scanning electron beam and communicates the timing of this event to the computer. At the end the position of the pen on the monitor surface can be calculated with astonishing accuracy.
Even if it was a desaster from an ergonomics point of view, the Light-Pen opened the door to a new family of computer applications: Computer Aided Design software.