Most systems that harvest energy from movement use piezoelectrics. That's fine, there's nothing wrong with that, except that piezoelectrics generally produce power measured in milliwatts. Milliwats of power can potentially run low-power sensors, trickle charge a battery, or (if you're lucky) run something like an iPod shuffle.
Researchers at UW-Madison have been working on an energy harvester that they say can deliver watts of power. And not just one or two watts, but up to 10 or 20. This is a huge amount of power to get from a "free" energy harvester; we're talking thousands of times more powerful than the current generation of pizeoelectrics. An iPhone, for example, typically consumes under five watts. This means that 10 watts could power two iPhones, and 20 watts could power four of them! Four iPhones! Imagine the possibilities!
To get this much power, the researchers ditched the piezoelectric idea entirely, and instead invented a new energy harvesting processes they're calling "reverse electrowetting." A liquid metal (in this case, it's something called galinstan which is non-toxic and used in thermometers) is stored in pouches at the heel and sole of a shoe. As you walk, you pump the nano-sized droplets of the liquid metal through tiny channels, creating a electricity which is stored in a battery at the center of the shoe. It's a completely sealed system that requires no maintenance: all you have to do is walk.
To take advantage of all this power, one option is to just kludge a USB port into your shoe and plug in directly, but there may be more creative ways to go about making your devices last longer. For example, your cellphone expends much of its power broadcasting intensively enough to reach the closest cell tower. If instead you had a sort of miniature self-powered cell tower in your shoe that could amplify you're phone's signal, your phone would only have to broadcast a few feet instead of tens of miles, boosting its battery life by a factor of 10 or more.
The company that the UW-Madison researchers founded to commercialize this technology is called InStep NanoPower. They're currently working on a prototype which should be ready in a few years, and a product for both the military and civilian markets will follow. Ultimately, the cost of the embedded harvester is not expected to exceed the cost of the footwear itself, meaning that at the outside, a pair of shoes might get twice as expensive with the harvester inside. But considering how much power this thing can supposedly generate, I'd say that it would definitely be worth the premium to never have to worry about recharging your stuff again.
As long as you get off your ass and walk around every once in a while, that is.