Namecoin

Namecoin is a decentralized key-value database. It has an internal currency too, called Namecoins. Namecoin uses the blockchain data structure and the proof-of-work consensus protocol.

In Namecoin, you can store key-value pairs of data. To register a key-value pair, you need to spend Namecoins. Once you register, you need to update it once in every 35,999 blocks; otherwise, the value associated with the key will expire. To update, you need Namecoins as well. There is no need to renew the keys; that is, you don't need to spend any Namecoins to keep the key after you have registered it.

Namecoin has a namespace feature that allows users to organize different kinds of keys. Anyone can create namespaces or use existing ones to organize keys.

Some of the most popular namespaces are a (application specific data), d (domain name specifications), ds (secure domain name), id (identity), is (secure identity), p (product), and so on.