It's called Skinput, and here's how it works: The user wears an armband, which contains a very small projector that projects a menu or keypad onto a person's hand or forearm. The armband also contains an acoustic sensor. Why? Because when you tap different parts of your body, it makes unique sounds based on the area's bone density, soft tissue, joints and other factors.
I thought of a lot of things to do with a pico projector, I have one for my I phone. Heads up displays of all types is an obvious one like on your eye glasses. But Skinput now that is revolutionary.
Tap Your Skin to Dial Your Phone