Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Spring Security - The Spring portfolio has many projects, including Spring Framework, Spring IO Platform, Spring Cloud, Spring Boot, Spring Data, Spring Security...
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Firebase Authentication - Application and Data, Application Utilities, and User Management and Authentication
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.