Touchless is an open-source application framework that enables you to use your webcam as a multi-touch interface.
"Right now the Touchless Demo lets you play with four proof-of-concept ideas: Draw, Image, Snake, and Defend. The first is a free-form drawing application, while Image is an image manipulation utility that allows you to zoom in or out and move around on a map with marker gestures. The other two are games (Snake is exactly like the classic, and Defend is up to four-person Pong). To set up a marker that Touchless tracks, just grab something colorful, click Add A New Marker, then draw a circle around the object. From there on out, Touchless will monitor that marker wherever it is in the shot. Right now the application is a little clunky, but as a proof-of-concept it's not bad (and it's fun to play with)."