I like using Mozilla's Persona library https://login.persona.org/ https://login.persona.org/. Very easy to integrate, you actually get the username they registe

I like using Mozilla's Persona library https://login.persona.org/ https://login.persona.org/. Very easy to integrate, you actually get the username they registered with, and (IMO) it doesn't have the trust baggage that comes with using Facebook/Twitter logins