Ambient backscatter
 

...or, communications without batteries!

http://www.nbcnews.com/technology/no...ves-6C10910339

I wonder if we will see this kind of application in our lifetimes. Imagine traveling from place to place without having to worry about batteries (for at least small devices) anymore...

One caveat: This technology needs some kind of radio emissions to function, so I don't think you'd be able to make it work in extremely remote areas.

Somewhat related invention using a technique patented in 1941 and deployed in 1945 by the Soviet Union. It hung in the US Embassy in Moscow until it was discovered in 1952: http://en.wikipedia.org/wiki/Thing_(listening_device)