OpenLDAP is a project to implement an LDAP server.
Versions for common linux distributions have been released, as well as versions for BSD distributions and OSX, Windows and various Unixes.
OpenLDAP consists of 4 pieces: an LDAP server daemon, an LDAP replication daemon, development libraries, and miscellaneous tools and utilities for working with LDAP and OpenLDAP.
SourceLabs covers OpenLDAP in its Self Support for Linux and Open Source Java offering.