Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
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.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
ClearOS - A very good alternative for Windows Server, for free