Keycloak - Open Source Identity and Access Management for modern Applications and Services.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
389 Directory Server - Enterprise-class open source Linux LDAP server.
Microsoft Active Directory - Active Directory is a special-purpose database \ 8212; it is not a registry replacement.
Samba - Samba serves as an open source alternative to the traditional domain controller offered by Microsoft.