1. Google Identity Platform.
https://developers.google.com/identity/
2. Facebook Login.
https://developers.facebook.com/docs/facebook-login/
3. OpenID.
https://zh.wikipedia.org/wiki/OpenID
4. bankID.
https://www.bankid.no/en/company/
5. Claim-Based Identity.
https://en.wikipedia.org/wiki/Claims-based_identity