Your users have enough of creating another account - just for your website. Let them log in to your Joomla!-powered website using the credentials of their favorite social service (Facebook, Twitter, LinkedIn, MySpace, PayPal, OpenID, and many more).
Prasi is an authentication framework with full user data management for Perl or PHP web applications. Enabled services can save and share user profile data and can be on different servers. Available in 10 languages, compatible with OpenID.
OpenID is a decentralized authentication protocol for web applications. OpenID users need share credentials with only one OpenID provider, and not every forum and network they log on to. This leads to a safer and more convenient Web. http://openid.net/