Introduction of Custom OAuth providers
Starting with Umbraco 7.3.1 Umbraco uses ASP.Net Identity for authentication of backoffice users. Asp.net identity is a very flexible and extensible framework for authentication.
Out of the box Umbraco ships with a ASP.Net Identity implementation which uses Umbraco's database data. Normally this is fine for most Umbraco developers but in some cases the authentication process needs to be customized.
For more information on connecting other oauth providers look at the Identity Extensions package.