Google just made it easier for people with Yahoo accounts to sign up for a Google account. With one click, you can now use your Yahoo credentials to sign up for a Google account such as Gmail, Google Docs, Google Reader, or even AdWords. The one-click sign-up is done using OpenID, which both Google and Yahoo support. It also uses the OAuth authentication method quickly becoming the standard across the Web (it is the same one Twitter uses with third party apps and sites).
The idea is that instead of signing in with your Yahoo ID, and then clicking off to your Yahoo mail to click on a verification link, a button just takes you to a sign-in page on Yahoo, which verifies your account to Google, and then sends you back. It is a much more civilized way to sign into a site using an existing ID. Google first combined OpenID and OAuth back in January, 2009 with Plaxo on a test basis.
It is trying this method out with Yahoo first, and hopes to expand it to other OpenID partners such as Microsoft. The one-click method results in more people completing the sign-up process. If you don’t already have a Google ID, chances are you have a Yahoo or Microsoft one, so targeting users from those two makes sense. (Both Yahoo Mail and Hotmail still have more users than Gmail).