NameID is a system to register online identities in a decentralised fashion via the Namecoin blockchain. This allows, among other things, for password-less logins to web services. This plugin makes it easy to do that. It recognises login forms of compatible web sites and handles the login protocol seemlessly. The first site to accept such logins is https://nameid.org/, which can be used as an OpenID provider to log into non-compatible sites as well (as long as they support OpenID).
Note that an installed Namecoin client is required.