Today Google announced integration with Yahoo OpenID. Yahoo users can now sign up for a Google Account with their Yahoo email credentials. Instead of the old fashioned way where you would have to verify your email account by logging back into Yahoo, you can just verify with your login credentials.
Once you verify your Yahoo Account with Google you have access to all Google services and apps. This is great for users that want to try out Gmail or another Google App without much trouble or users that want to switch to Google all together. Google plans to roll out this service to other email providers in the future such as Hotmail. More information in the source.