OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...
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.
Univention Corporate Server - Univention Corporate Server UCS with Active Directory functions and App Center is the cost-efficient and popular successor to Microsoft's server solutions.
OpenDJ - OpenDJ is a directory server