I just read about a new sidewalk design being used at the London Olympics that converts footsteps to electricity. These recycled rubber slabs made by PaveGen compress when stepped on, and the kinetic energy is converted to electricity (the company won't reveal the exact secret to how it works). The system has been in testing in England, including at a school where the kids apparently step on the slabs like it's their job. The developers envision that the sidewalks will be used primarily for outdoor lighting and similar infrastructure, but they could also be used for short-term projects like powering cell phones at music festivals (it takes about 25 steps to charge a phone -- not bad).
You aren't likely to see these sidewalks anytime soon -- they're cost-prohibitive for most applications at this point -- but hopefully this kind of technology will gain, uh, traction and someday help us to further reduce our reliance on traditional sources of electricity. For more information, check out the PaveGen website or the CNN article where I first learned about this innovative use of the bottoms of our shoes.