Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
CAS - Apereo CAS - Enterprise Single Sign On for the Web
Samba - Samba serves as an open source alternative to the traditional domain controller offered by Microsoft.