Openid Introduction

OpenID is the decentralized, lightweight protocol for single sign-on and portable identity that is causing a massive transformation in today's internet. More than 25,000 Web sites currently accept OpenID, with double digit month over month growth. Many of the world's leading companies, including AOL, Yahoo!, MySpace, Google Blogger, France Telecom, Telecom Italia have adopted or announced support for OpenID enabling over 500 million users.

This Open Source technology lets you use your existing presence on the web as a form of identity. It was designed to solve real-world problems around single sign-on, without unnecessary complexity. Just like you can use an email address as a form of identity, OpenID lets you use a web address (URL) that you control to sign in on other sites. And just like email addresses, you can have more than one, for work, or at home, or for any other use. But unlike email, the sites you access can't spam you or access your data unless you give them permission.

in reference to: JanRain - OpenID (view on Google Sidewiki)