Persona is a nice way to login via email clickthrough that's not bound to any specific email provider or mobile app, automatically turns into an OpenID or OAuth login if the email provider supports that, and supports an alternative login method via an open standard that can be used by the browser to log you in without any kind of interaction (only implemented by Firefox though). Sadly it never caught on (presumably because there is no business model behind it, unlike locking the user in to a mobile app or federated login via a provider that then sells the user data).