In some parts of the world, getting water to people is a huge problem. The water is usually a fair distance away from the village and possibly contaminated. Even though there is water under the ground, they don't have the electricity or full necessary to pump it to the surface. PlayPumps came up with an amazingly clever solution. Instead of trying to lug a big diesel engine in constant need of fuel to each village, they looked for possible sources of energy that were already there. They decided to use the energy of children's play to collect water.
It's a good reminder to creative folks, instead of introducing a new element to solve a problem, have you really used what's already there?