Proof of work was the first, crude consensus algorithm for crypto-currencies. A lot of more eco-friendly consensus algorithms have been proposed. Ethereum, the second largest crypto-currency in the world, is currently moving towards a Proof-of-stake consensus algorithm, instead of the old proof-of-work one.

Don't forget that decentralized currency is a rather new technology, and is constantly seeing progress being made on all fronts. You can even mine Grid coins doing SETI and Protein folding work!