Proof of stake is an alternative that inverted the trick that rewarded effort with a coin. Instead of using a coin: whose value is proportionate to the system's, to reward the effort, PoS uses the coin instead of the effort. You ask around and you trust the message backed by the side with most coins.

The advantage of PoS is that it requires less effort and costs, by definition. The advantage of Proof of Work is that it's more inclusive as anyone who makes the effort may be rewarded with more coins.