CryptoKitties is the first decentralized open-source game in the world that popularized NFTs, developed on the Ethereum blockchain by Dapper Labs in 2017. Although it had a slow start to adoption, it quickly gained momentum during the NFT hype cycle and has had over 1.5 million users using the platform. Players can collect, breed, buy, and sell cats that vary in rarity and appearance. Each cat in the game is an NFT, so players can make a profit inside the game or on NFT marketplaces.