An interesting open-source technology that allows you to use a laser pointer, combined with a projector and webcam, to "Grafitti" onto any wall.  This could be useful in a classroom where the teacher can annotate the slides from afar.  The way it works is that a webcam is mounted above the projector, it watches for laser dots on the screen, then renders some "virtual paint" at that location.

I wonder if it works with multiple dots in different colors, all annotating simultaneously?