Touchless by Microsoft

Touchless enables touch without touching by using a webcam to track color based markers. Touchless includes two parts:

-Touchless Demo is an open source application that anyone with a webcam can use to experience multi-touch, no geekiness required. There are 4 fun demos: Snake - where you control a snake with a marker, Defender - up to 4 player version of a pong-like game, Map - where you can rotate, zoom, and move a map using 2 markers, and Draw - the marker is used to guess what.... draw!

-Touchless SDK is an open source SDK that enables developers to create multi-touch based applications using a webcam for input, geekiness recommended.

This project is an Office Labs community project, which means a Microsoft employee worked on this in their spare time. It is also an open source project, which means that anyone can view, use, and contribute to the code.