CryptoKitties was the first popular blockchain game. It was released 2017 on the Ethereum blockchain. At its peak, it was so popular that it clogged the whole Ethereum network.
CryptoKitties made its first appearance in October 2017 at an Ethereum hackathon. The team behind it is known as Axiom Zen. Of course, they won the hackathon with this unique project. It is a collectible game, with the addition of breeding.
The ownership of a CryptoKitten is tracked via a smart contract on the Ethereum blockchain. Each cat has a unique number and a 256-bit genome which defines its appearance (e.g. shape, eye color, base color, accent color, fur, etc.). This genome can create up to 4-billion possible variations. That makes each cat unique and some combinations more desirable than others. Because CryptoKitten is ERC#721 tokens, they can be bought, sold & transferred with the security of the Ethereum blockchain.
Two cats can breed together to produce an offspring, which is a combination of the visual attribute of the two parents. One of the parents then has a cooldown-period, before it can breed again. This cooldown-period increases each time, to prevent the creation of too many cats. The newborn cat will then be assigned to the female parent cat. After that, both parents can breed again.
On November 22th, the final CryptoKitties smart contract was deployed on the Ethereum blockchain.
On November 23th, CryptoKitties was already under the Top 3 most active Ethereum contracts.
On December 5, CryptoKitties had already 60,000 registered users. Over 100,000 cats have been created. And over $5,000,000 USD worth of transactions have been processed. At this date, CryptoKitties were already responsible for around 25% of the Ethereum traffic.
In December 2017 the hype reached its peak and one kitten sold for around 247 ETH (around $120,000 back then). At the same time, Etherscan reported 6x more pending transaction than usual. This affected the whole network.
In January 2018, CryptoKitties had over 250,000 users.
With the general crypto downward trend in early 2018, the CryptoKitten hype cooled down as well. Although a new kitten was created every 15 minutes until November 2018.
As of today, the game is still running. The company behind the website cryptokitties.co is Dapper Labs Inc. To start you have to buy a CryptoKittie with ETH. If you have two or more, you can still breed & sell/gift them.