From TechCrunch:

Gleaning electricity from radio signals isn’t super-efficient (or a new concept), but it’s possible technically. Two companies from Japan have recently come up with new devices that can do just that: one is harvesting energy from cell phone signals, the other uses a rectifying antenna (Rectenna) to produce energy.

First, a device [JP] recently announced by Tokyo Electron Device can pick up radio signals from cell phones in the 800MHz band and convert them into electricity (800MHz is the frequency used in handsets in Japan). The company says that users can expect to get up to 2.5 milliamperes of charge (stored in a small lithium-ion battery).

The device collects signals from other wireless gadgets, too, and it can provide a few more minutes of run time when built into tablets and notebooks (which is better than nothing). Tokyo Electron markets it as “green” product and cooperated with US company Powercast in its development.