Blogroll for tag authenication
OAuth - An security authentication protocol that provide a simple and standard API for web applications to interact
OpenID - An open, decentralized, free framework for user-centric digital identity, kind of like MS Passport for Open source
OpenID for non-SuperUsers (Sam Ruby)
phpMyID - A PHP OpenID library
OpenID For PHP - A OpenID 2.0 Compliant PHP5 Library