Write some service and learned about the existence of such thing as rpxnow.com.
It allows you to hang it on a website widget with methods login (facebook, google, etc.) from which the user can choose convenient for him. The developer (ie, me) remains to obtain a unique identifier and user name (as well as additional information depending on user-selected method). Quite convenient, however, revealed a number of weaknesses (ranging from inability to somehow configure the widget's appearance and ending with the too frequent need for re-authorization).
So I wonder whether there are analogues.