External Authentication (OAuth)

You can plugin pretty much any 3rd party OAuth provider for authentication purposes. External OAuth logins with UmbracoIdentity are the exact same principles as with any standard ASP.Net Identity procedures.

Some additional info on some OAuth providers:

Azure AD

Here's an integration guide:

https://shazwazza.com/post/configuring-azure-active-directory-login-with-umbraco-members/

Azure AD B2C

Here's an integration guide:

https://azure.microsoft.com/en-us/documentation/articles/active-directory-b2c-devquickstarts-web-dotnet/

Google

Google by default does not allow OAuth2 support any longer without credentials, so this call

 app.UseGoogleAuthentication();

will not work. You can however force it to work for testing by setting up your own Google app and getting a custom clientId and clientSecret. Follow this guide

Facebook

Here's an integration guide:

http://code.msdn.microsoft.com/windowsdesktop/MVC5-Authentication-App-b5200efd

You can create a FB app here:

https://developers.facebook.com/

Identity Server

There's an old communnity Nuget package to help with integration https://www.nuget.org/packages/UmbracoIdentity.IdentityServer but unfortunately it seems like it's no longer maintained, however the package and source code could still work or help https://github.com/stokesy43/UmbracoIdentity.IdentityServer