389 Directory Server!

OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.The suite includes:slapd - stand-alone LDAP daemon (server)libraries implementing the LDAP protocol, andutilities, tools, and sample clients.Also available from the OpenLDAP Project:Fortress - Role-based identity access management Java SDKJLDAP - LDAP Class Libraries for JavaJDBC-LDAP - Java JDBC - LDAP Bridge Driver

FreeIPA is an integrated Identity and Authentication solution for Linux/UNIX networked environments. A FreeIPA server provides centralized authentication, authorization and account information by storing data about user, groups, hosts and other objects necessary to manage the security aspects of a network of computers.

Active Directory (AD) is a directory service that Microsoft developed for Windows domain networks and included in most Windows Server operating systems as a set of processes and services.

ApacheDS™ is an extensible and embeddable directory server entirely written in Java, which has been certified LDAPv3 compatible by the Open Group. Besides LDAP it supports Kerberos 5 and the Change Password Protocol. It has been designed to introduce triggers, stored procedures, queues and views to the world of LDAP which has lacked these rich constructs.

It's an active directory replacement built on CentOS + Samaba4. Includes support for Directory, DNS, DCHP, and NTP type services.