CryptoKitties is an oldy but a goody – the first game based around ERC-721 collectibles on the Ethereum blockchain. Buy kitties with Ether and start breeding them; each has a unique phenotype and genotype that combines into offspring kitties, with four billion possible cattribute outcomes. Some outcomes are more valuable than others; you are the sole owner of your kitty's code, so if its a good one, go ahead and sell!
Every 15 minutes, a new gen-0 cryptokitten is born. This will continue until November 2018, when the 50,000 gen-0 kitty cap is reached. Until that fateful day arrives, you can get MetaMask wallet, put some ETH in there and head over to the Marketplace on the Cryptokitties website to bid on a brand new kitten. Alternatively, you can just buy one from a trader. Once you have your kitties, you can breed away. The Cat Codex will track your progress towards collecting all the available cattributes. You can breed and sell directly on your CryptoKitty's profile page.
Although there will only ever be 50,000 gen-0 kitties, the total number of kitties is potentially infinite. The fun of it is that certain specific cattribute combinations, plus the cat's generation number and cooldown, can make some kitties incredibly valuable on the market. Gen-0 cats are the most desireable – the higher the generation number, the worse the kitty will do. The cooldown, on the other hand, is kind of like a refractory period: the amount of time a kitty needs before it can breed again after the last time it gave birth. Like with refractory periods, faster is better. But the cooldown of each kitty starts to rise with each birth, depreciating the poor guy.
Look out for mewtations in your CryptoKitty's family tree – they can award you Family Jewels! There's also something called a Fancy Cat: rare, non-unique cats with custom art.