Probably Namecoin, but there are several now. Probably ENS (Ethereum name service) is the most notable.

But keep in mind Zooko's triangle - you can't have all three of secure, human-readable, and decentralised.

In Zooko terms, blockchains are secure, human-readable, centralised registries - there is only one, and you have to stay connected to it. Onion domains (which are public key hashes) are secure, unreadable, and decentralised. Petnames (as used in I2P) are insecure, readable, and decentralised.