Chris Harrison and Scott Hudson (Carnegie Mellon University) came up with a way to use any flat surface as a gesture based input device (Scratch input). An absolute breakthrough which makes extraordinary interactions, like for example controlling your television or music player with your wooden kitchen table, possible.
Scratch input is specifically designed to use augmented existing, passive surfaces as an input device. Using a simple sensor which is sticked on the surface, the sound-waves are captured. Due the fact that every movement produces a different sound-pattern, gestures can be recognized.
Interesting about it is that in a lot of cases this will make an interface redundant. And if you do need one, you can design one that looks and feels much more natural and organic.
I could go on explaining how it works but this video does a much better job at that:
What has been explained in the video, and a bit more, can also be found in the paper they published which can be downloaded here.