Exercise is the deliberate use of your body's chemical energy, which encourages your muscles to be stronger and uses up the fat stores. But usually, the energy from this is thrown away.
I'm imagining an exercise bike (or this could be adapted to a weight-lifting machine, cardio device, or what have you), that charges up a car battery. One would do an hour or two of exercise, charging the battery at 100 watts. Humans generally use about 150 watts, but I imagine that some energy will be lost in the conversion process. After exercising, one could connect the battery to your house, and for a brief time, your appliances would be charged with human power.
Or this could be used in jails, with inmates charging batteries that power the jail, and perhaps ultimately, society at large. Finally paying back what they stole in the first place, in a way both very direct and indirect. (Direct because they're making electricity from their own efforts, but indirect because this energy...would be sold for public benefit to pay for the victim's needs?)