OpenID

OpenID is an open standard and decentralized authentication .

Promoted by the non-profit OpenID Foundation, it allows users to be authenticated by co-operating sites (known as Relying Parties or RP) using a third party service, eliminating the need for webmasters to provide their own ad hoc login systems, and allowing users to log in to multiple unrelated websites without having to have a separate identity and password for each.

Several large organizations either issue or accept OpenIDs on their websites according to the OpenID Foundation:AOL, Blogger, Flickr, France Telecom, Google, Amazon.com, Hyves, LiveJournal, Microsoft (provider name Microsoft account), Mixi, Myspace, Novell, Orange, Sears, Sun, Telecom Italia, Universal Music Group, VeriSign, WordPress, Yahoo!, the BBC,IBM,PayPal, and Steam, although some of those organizations also have their own authentication management. Facebook did use OpenID in the past, but moved to Facebook Connect.

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Wikipedia