update : 23/12/2020 OpenLDAP

OpenLDAP is an open source company directory. It is a sound alternative to commercial directories. OpenLDAP was created in 1998 by Kurt Zeilenga.

OpenLDAP implements the most recent version of the LDAP protocol (v3), while providing a scalable architecture through an overlay system and back end. Each overlay provides additional functionalities (dynamic group, access log, password policy, etc.). In addition, directory data can be stored in different back ends (LDAP proxy, query transfers to Perl or Shell scripts, or a SQL database, etc.). OpenLDAP also incorporates replication and delegation mechanisms, enabling for example the implementation of an LDAP directory distributed to multiple sites, with each one administrating its own branch.
OpenLDAP is distributed under the OpenLDAP Public License, is compatible with the GPL and has been validated by the Free Software Foundation.
OpenLDAP is developed in C and runs on Linux, on different BSD variants on UNIX based OSs (Android, AIX, HP-UX, Solaris, etc.), as well as on Windows and Mac OS X.