Touchscreens aren't exactly expensive or rare, but the uTouch project from researchers at the University of Washington is doing something we've never seen before: creating touchscreens by mapping your house's unique electromagnetic interference signature.
This is pretty complicated, so I'll let ExtremeTech explain it:
So if you use the uTouch project's electromagnetic signature sensor--the team doesn't specify exactly what's in that, though electromagnetic interference can be detected by infrared or spectroscopy--basically any electronic item plugged into your house's electrical system can serve as a touchscreen (or touch-whatever). Screens are ideal, because they're flat surfaces upon which are thousands of little sensors.
The system isn't particularly detailed; think basic selecting gestures like tapping with one or several fingers rather than complex gestures like you'd perform on an iPad. But it's certainly a cheaper solution for situations where you don't need that much detail, like, say, huge displays at a museum. And it has one very cool trick: the sensor itself, since it maps the signature of your entire houses and the changes therein, doesn't actually need to be hooked up to the screen you'd like to touch. In fact, it could be across the house or in a basement somewhere.