Check this video out. It’s a digital paint brush. Basically, there’s a camera in the brush that photgraphs/records what you hover it overm then the screen traces the movements across itself and allows you to “paint” with the pattern or image you just photographed. You simply have to see this to understand what I’m saying. It’s useless, but I know I’d still love to play around with it.

http://web.media.mit.edu/~kimiko/iobrush/iobrush_quicktime_small.mov

You need Quicktime installed to view it