In one of the greatest moves since the Zimbra acquisition, Yahoo announced today that it will provide support for OpenID 2.0 at the end of this month. "Yahoo! (...) announced its support for the OpenID 2.0 digital identity framework for all 248 million active registered Yahoo! users worldwide. OpenID, an open framework based on proven Internet technologies, enables users to consolidate their Internet identity, eliminating the need to create separate IDs and logins at all of the various websites, blogs, and profile pages they may visit in the course of their online session. In addition to the many leading Yahoo! services users already enjoy, anyone with a Yahoo! ID will be able to use the same ID for easy access to any sites that support OpenID 2.0. Yahoo!'s initial OpenID service, which will be available in public beta on January 30, enables a seamless and transparent web experience by allowing users to use their custom OpenID identifier on me.yahoo.com or to simply type in www.yahoo.com or www.flickr.com on any site that supports OpenID 2.0."
From January 30, any Yahoo ID will also become an OpenID that could be used to authenticate on any site that accepts OpenIDs (for example, to post comments on Blogger). Even if many important companies backed this authentication system, very few started to support it and this reduced its usefulness. Yahoo mentions that it will triple the number of OpenIDs to 368 million, although I don't think there's a way to calculate the total number of OpenIDs.
Hopefully this is just a start and many other companies, including Google, will become OpenID providers and will accept OpenIDs.