When you run your fingernail over a hard surface like wood or drywall, it produces a high-frequency sound. It's barely audible to the naked ear (unless the surface happens to be a chalkboard), but the sound happens to be very distinctive, and it measurably changes when when you do it faster or with more pressure. This is the idea behind a scratch input — a system that would measure surface scratches and interpret them as commands.
Researchers at Carnegie Mellon University are developing such an interface. In theory, all you'd need to turn any surface in to an input device would be a kind of stethoscope and a processor to interpret the scratches. The potential applications are innumerable: Scratch your computer to turn it on, or use your desk as a drawing surface for your fingernail, with the drawings appearing on the monitor. Check out the video below to see some more examples.
What else would you use a scratch input for? No answers from housecats, please.