TreeHugger has shown all kinds of generators for bicycles, but here is a novel approach that would do well where I ride. Industrial designer Deco Goodman takes advantage of our crumbling infrastructure and pothole-filled roads by installing a piezo-electric generator in a shock absorber built into the seat post. Every tooth-jarring bump now is a little bit softer as the energy is converted to electricity and stored in the battery.
He also gets power out of a generator on his rear wheel, where he turns the whole wheel into a big rotor.
I would have thought it would be more effective if mounted in the front forks, but still it is a clever way of making a virtue of the necessity of riding on bumpy city streets. Also using the seat would let a cyclist retrofit an old junker like this.