I've thought before about all the energy wasted around our country in gyms.  Especially on the bicycles and treadmills. That's an immense amount of energy going no where, one spin class probably puts off enough to charge quite a bit.  I recall watching something where they were sending laptop computers to 3rd world countries that could be charged by handcranking it. Figure out a way to hook some batteries up to the cardio machines and you could turn those gyms green.  Not to mention all the people pushing and pulling around weights that could be using that energy to actually do something productive. Every cable gym machine could somehow be harnessing the energy the human body is expending.