I think anyone that has spent at least 20 minutes at the gym has wondered, "can't we convert all this effort and pain we're exerting (and the sweat that is pouring down our bodies) into usable energy?"
My sister and I had this thought years ago, and I'm happy to find out that there's a project out there working on just that : converting exercise bikes into a source of electricity.
The company's called The Green Revolution.
Green Revolution has developed a technology that integrates a generator into each bike, and the electricity produced is then re-distributed back into the power grid. They are currently focused on health clubs and gyms in order to benefit from economies of scale - in other words, having multiple bikes running at the same time to feed the electricity back into the system. For instance, a class of 20 bikes will produce about 3kW. That's more than enough electricity to light the gym ! (if you have say, 60 50W bulbs)
Green Revolution currently provides its technology for 75 gyms in the US and they would love to branch into the European market (as they claim that Europeans are much more conscious about their carbon-footprint than Americans are) and they are working to create a European version that meets all required regulations and certifications by the end of 2011, early 2012 at the latest.