I made a multipurpose transparent desk that has the ability to sense the hand and finger positions of anybody who touches it. This makes it a very interesting input device for your computer as it can act as your keyboard, sketch board, musical instrument, etc.
The sensing method is based on a technology called Frustrated Total Internal Reflection. Basically you flood a transparent piece of acrylic with IR light and then you place a camera with a visible light filter beneath it.
When anything is placed on the acrylic, it bends it slightly which changes the reflection properties and IR light “escapes” the acrylic. This is sensed by the camera.
When the camera has sensed the IR light in the acrylic, tracking it becomes a software problem. I used OpenCV for blob-tracking.