SolarCoin (SLR) uses a secondary proof of work mechanism that incentivises uptake of clean, renewable energy! In this case, solar energy specifically.
It is a scrypt-based descendent of Litecoin. Simply by mining and using the coins as you would any other digital currency, you are contributing to the development of solar energy. The creation of these new, renewable energy sources by SolarCoin helps give the coin an almost carbon neutral footprint in its mining, unlike other major coins.