Years ago, I knew OpenId.
Years ago, I knew Google account supports OpenId as OpenId provider.
Years ago, I knew Google apps premium version account support SSO.
And I was misled by many threads that Google apps standard version account does not support OpenId. I did try several times without success, but I did not try hard.
Today, a great post How to Setup OpenID with Google Apps gave me a big confidence that I can do it. So I tried hard – spent a whole evening trying, and successfully logged into SourceForge with my Google apps acount, and I am using Google apps standard version!