Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Firebase Authentication - Application and Data, Application Utilities, and User Management and Authentication
LDAP - LDAP is a very flexible standards-based method that can be used to store and query information about users, groups, devices and provide authentication and authorization services in a networked environment.
OmniAuth - OmniAuth is a flexible authentication system utilizing Rack middleware.
OAuth2 - Application and Data, Application Utilities, and User Management and Authentication
Spring Security - The Spring portfolio has many projects, including Spring Framework, Spring IO Platform, Spring Cloud, Spring Boot, Spring Data, Spring Security...