OpenLDAP
OpenLDAP Software is a free, open source implementation of the Lightweight Directory Access Protocol (LDAP) developed by the OpenLDAP Project. It is released under its own BSD-style license called the OpenLDAP Public License. LDAP is a platform-independent protocol. Several common Linux distributions include OpenLDAP Software for LDAP support. The software also runs on BSD-variants, as well as AIX, Android, HP-UX, Mac OS X, Solaris, Microsoft Windows (NT and derivatives, e.g. 2000, XP, Vista, Windows 7, etc.), and z/OS.
Read more about OpenLDAP: Project History and Core Team, Components of OpenLDAP Software, Other Modules, Release Summary, Replication