Namecoin is an open source decentralized key/value registration and transfer system based on Bitcointechnology (a decentralized cryptocurrency).

It allows you to:
Securely register and transfer arbitrary names (keys), no possible censorship!
Attach values (data) to the names (currently up to 520 bytes, will be extended)
Trade and transact namecoins, the digital currency NMC

There are plenty of possible use cases. Some examples:

• DNS: domain_name => domain_zone_configuration (also for i2p, Tor)
• TLS: https like encryption and secure identification by Namecoin domain/ID/name
• Alias/Identity: user_name => user_public_identity (email, name, gpg key, BTC-adress, etc)
• Timestamping: document_hash => document_infos (name, hash, owner, etc)
• Broadcasting / Messaging
• Web of trust
• Bonds, shares
• Voting
• Torrent tracker

To register a name, you must own some namecoins (NMC, the internal cryptocurrency used by the software).

Find more information in the dot-bit wiki.