My main problem with these (as user) is that I usually have hard time remembering which sign-on system I have used for a particular site. Twitter? Google? Faceb

My main problem with these (as user) is that I usually have hard time remembering which sign-on system I have used for a particular site. Twitter? Google? Facebook? Username and pass? Amazon? In most cases what I would truly like to have is a option to order a sign-on link to my email that would be valid for single use and just for some minutes.