NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
ClearOS - A very good alternative for Windows Server, for free
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...
389 Directory Server - Enterprise-class open source Linux LDAP server.
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.