Piezoelectric generators take motion and turn it into electricity. They've been used to convert muscle movement into energy to run medical implants, but it's been hard to get them to scale up enough to power stuff that's bigger and more fun. Researchers at Georgia Tech have been able to create a tiny piezoelectric nanogenerator that's capable of powering an LED and a liquid crystal display, and your iPod is going to be next.
Piezoelectrics have been around forever in things like microphones, where they convert motion (vibrations) into electrical signals. Getting them to convert motion into more useful amounts of electricity is pretty tricky, since they tend to be rigid instead of flexible. Using nanotechnology, scientists have figured out how to embed tiny piezoelectric wires into flexible plastic and polymer sheets, which can generate power from being squeezed.
Georgia Tech's nanogenerator measures just over a half-inch square, and compressing it by four percent once a second generates 11 milliwatts. This is easily enough to power a calculator, and although something like an iPod requires about 80 milliwatts, at this point, it's just a matter of scaling up the technology (or using more nanogenerators).
The nanogenerators are flexible enough that it's possible to embed them in fabric, where they'll happily produce little squirts of power every time they move. If you work them into socks, or shoes, or even a jacket, they should be able to trickle charge all of your personal electronics without much trouble at all, as long as you don't spend your entire life on the couch.