Proof-of-play is a blockchain consensus mechanism in which new transactions are confirmed by players who are successful at a game built into the protocol. As they are rewarded with cryptocurrency for doing so, all proof-of-play is theoretically a form of human mining. These games are highly decentralized by their nature, with players effectively competing via peer-to-peer network.
The earliest and still only real example of this is Motocoin. Proof-of-play blockchains are susceptible to network takeovers known as 51% attacks if one player becomes too good at the game (enough to win multiple times—and thus mine multiple blocks—in a row). In reality, this is likely to be accomplished by AI bots, effectively giving network control to AI developers instead of players, as happened with Motocoin.