Super Efficient Energy?

Where is the production of energy so efficient it might as well be free?

Simple, look for systems and processes that are in motion, use your form of resistance in order to capture that potential energy and convert it to your benefit. This is the same process that dams, solar-cells, and windmills use in order to capture an energy that is always in motion. Wouldn't it be nice to capture the potential energy of tornados, earthquakes, volcanos, and tsunamis? Hmm, guess that's too far in the future. Maybe it need only be something as simple as capturing the rain.