Gravity - Potential Energy
In the case of the Earth, our planet is falling into the valley of space-time created by the incredible mass of the Sun - the Gravitational Pull. And as Earth is trying to travel in a straight line perpendicular to the Sun - the Countering Velocity - maintains Earth caught in an orbit around the Sun. And what is true for the Sun and Earth, is true for all the planets in the solar system, the solar system within our galaxy, and all galaxies within the universe.
Rain clouds on Earth, or the water at the top of the water-fall, for instance, have Gravitational Potential Energy - GPE - (usually noted as Ug). The amount of energy, depends on the mass, and height above the surface of the Earth. When the rain falls from the sky for example, the GPE is transformed into kinetic energy, and can be transformed to do work.
Some coal-powered power stations pump water up into high dams in the mountains, during low energy demand, to store the potential energy until periods of high demand. When the stored water is released, it can be used to power hydroelectric generators. Innovative integrated wind-power with hydroelectric, or wind-hydro hybrid systems are also starting to be used and developed. Regenerative Agricultural design systems like Keyline Design®, can also use the topology of the land to catch water running off the landscape into catchment dams - at the highest points (the keypoint) on the landscape. By catching the water at the highest point, the water has the highest potential (on the landscape) to do work without pumps - and so the highest possible water pressure in any pipe systems leading from independent or inter-connected dams.
This company is using gravitational energy to develop battery less lights (so no storage or life limit).