Raphael writes “I wanted to be able to control my music player, xmms, using a laser pointer and a wall. I drew the symbols for rewind, stop, play, forward on paper sheets using fluorescent ink so it looks cool with a blacklight and sticked them on the wall. I coded a simple tool that monitors the wall for the presence of a small spot using a webcam. When the spot appears somewhere, if it is inside a defined hotspot, the appropriate command is executed”. Link.