Google has an interesting hybrid approach: http://sites.google.com/site/oauthgoog/UXFedLogin http://sites.google.com/site/oauthgoog/UXFedLogin If the user is l

Google has an interesting hybrid approach: http://sites.google.com/site/oauthgoog/UXFedLogin http://sites.google.com/site/oauthgoog/UXFedLogin If the user is logging in with an email account on a domain whose email is handled by an openid provider, it will switch over to logging you in with openid. Though I'm dubious though about their wording suggestion of "No, help me log in" for the situation where you don't need a password because you can use openid. A combination of your idea and Google's idea: let users login with only an email address. If the email's domain supports openid, use that, otherwise send an email with a link to log the user in.