OpenID

Overview
OpenID "is an open, decentralized, free framework for user-centric digital identity", according to their web site. In less jargon-laden terms, it allows users to use the same log-in (username/password) to access multiple web sites, without having to create a new account on each site. It also allows each of those accounts to be recognized as the same actual real-life user.

Reference

 * official web site

Code

 * phpMyID: a single user OpenID server written in PHP