Forbes traces the development of devices powered by body heat:
A resting male can put out between 100 and 120 watts of energy, in theory enough to power many of the electronics you use, such as your Nintendo Wii (14 watts), your cellphone (about 1 watt) and your laptop (45 watts). Eighty percent of body power is given off as excess heat.
…
Recent developments in nanotechnology engineering promise to usher in lots more body-powered devices. The basic technology behind the concept of turning body heat into electricity is a thermoelectric device. It is usually a thin conductive material that exploits the temperature difference between its two sides to generate electricity, known as the Seebeck effect. Such devices can work in reverse, meaning if you were to apply electricity to the device, one side would get extremely cold and the other extremely hot.…
Currently, a thermoelectric generator currently can only convert 0.4% of the heat energy into usable electrical power. With this efficiency, if you were to cover all of your body with thermoelectric generators you could produce 0.5 Watts of energy. This would feel extremely cold and would hardly be enough to power a cellphone.
…
MIT Professor Peter Hagelstein published a paper in November that showed a way to improve the efficiency of thermoelectric generators by up to 4 times in practice and up to 9 times in theory. Devices with that kind of efficiency could be used anywhere there is wasted heat–on the walls of power plants or lining the hoods of automobiles.