The first big site to allow users to sign in with Google accounts is Zoho. Users of the online office suite can now sign-in to create, collaborate, and share documents all without ever creating an account specifically for Zoho (where you can already sign in using your Yahoo account, btw). At the moment, Google is offering trial access to the APIs for enabling support on your own site. You can apply for access here.
In a blog post, Google writes, “Currently users are required to create individual passwords for many websites they visit, but users would prefer to avoid this step so they could visits websites more easily … If users could log into sites without needing another password, it would allow websites to provide a more personalized experience to their users.” Of course, this is the problem OpenID looks to address, and Google wants to help solve it, so long as the account your using to sign in elsewhere is a Google one.
Similar to the other companies that have embraced OpenID support, Google is simply now one of many identity providers. On the other hand, if you want to login to a Google service like Gmail, Docs, or Reader using OpenID, that is currently not supported. Yahoo’s implementation is quite similar to Google’s, and can be found on services like Plaxo in addition to Zoho.
Yes, a lot of people (and I'm sure most of our readers) have a Google account. But there is a bit of hypocrisy in saying you're moving to support single sign-on, when in reality, you're competing with the same old companies to be THE single sign-on provider, as opposed to providing single sign-on access on your own properties too.