If you head to Email Hosting, CRM, Project Management, Office Suite, Document Management and more and look at the login panel on your right, you can see they are accepting google sign-in as a login alternative for their visitors.

My question's how am I gonna do that for one of my sites? I have searched on google but it always turns out webmaster tools results rather than the thing I'm looking for.

I guess it will need an API so where can I acquire it?