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Server software

Apache Directory Server
Apple Open Directory
CA Directory from CA, Inc. (formerly eTrust Directory)
Fedora Directory Server (Red Hat Directory Server)
IBM Tivoli Directory Server
MXMS, from Atos Origin
M-Vault, from Isode Limited
Microsoft Active Directory
Novell eDirectory
OneLDAP An LDAP gateway to standard protocols like POP3 and IMAP.
OpenDS
OpenLDAP
Openwave LDAP Directory Server
Oracle Internet Directory
Penrose - a Java-based Virtual Directory Server.
repoze.who.plugins.ldap. Enables LDAP Authentication, and related utilities, in WSGI applications.
Siemens DirX
SIDVault
Sun Java System Directory Server 

Client software

Cross-platform

Apache Directory Studio: an LDAP browser and directory client for Linux, Mac OS X, and Microsoft Windows, and as a plug-in for the Eclipse development environment.
DirectoryMark, a benchmarking tool for LDAPv2 and v3 directory servers.
Ekiga (formerly GnomeMeeting) - open source VoIP and video conferencing application supporting LDAP and ILS directories.
JXplorer - A Java-based browser that runs in any operating environment.
LDAP Browser/Editor - A Java-based LDAP browser / editor.
Mozilla Thunderbird - an LDAP-aware email client.
Novell Evolution - an LDAP-aware PIM.
phpLDAPadmin - A web-based LDAP administration tool for creating and editing LDAP entries in any LDAP server.
SLAMD - an open-source load generation software suite, for testing multiple application protocols, including LDAP.
YALA - A general purpose web based LDAP administration GUI.

Linux/Unix

LDAPSoft LDAP Browser: a freeware proprietary LDAP directory browser for Linux
LDAPSoft LDAP Admin Tool: linux ldap administration client
Evolution - the contacts part of GNOME’s PIM can query LDAP servers.
KAddressBook - the address book application for the K Desktop Environment, capable of querying LDAP servers.

Mac OS X

Address Book - an LDAP-aware address book application built into Mac OS X.
Directory Access - a utility for configuring access to several types of directory servers, including LDAP; built into Mac OS X.

Microsoft Windows

LDAPSoft LDAP Browser: a freeware proprietary LDAP directory browser
LDAPSoft LDAP Admin Tool: ldap administration client
Softerra LDAP Administrator: LDAP administration client
Softerra LDAP Browser: a freeware proprietary LDAP directory browser
LDAPAdmin: a free, open-source LDAP directory browser and editor
MaXware Directory Explorer: a freeware proprietary LDAP directory browser
Active Directory Explorer: A free LDAP client tool from Microsoft (but originally from Sysinternals)

Among the LDAP/X.500 based implementations are:

eDirectory: This is Novell’s implementation of directory services. It supports multiple architectures including Windows, NetWare, Linux and several flavours of Unix and has long been used for user administration, configuration management, and software management. eDirectory has evolved into a central component in a broader range of Identity management products. It was previously known as Novell Directory Services.
Red Hat Directory Server: Red Hat released a directory service, that it acquired from AOL’s Netscape Security Solutions unit[1], as a commercial product running on top of Red Hat Enterprise Linux called Red Hat Directory Server and as part of Fedora Core called Fedora Directory Server.
Active Directory: Microsoft’s directory service is the Active Directory which is included in the Windows 2000 and Windows Server 2003 operating system versions.
Open Directory: Apple’s Mac OS X Server offers a directory service called Open Directory which integrates with many open standard protocols such as LDAP and Kerberos as well as proprietary directory solutions like Active Directory and eDirectory. It is a customized build of OpenLDAP.
Apache Directory Server: Apache Software Foundation offers a directory service called ApacheDS.
Oracle Internet Directory: (OID) is Oracle Corporation’s directory service, which is compatible with LDAP version 3.
CA Directory: CA Directory contains pre-caching engine which can index all attributes that are used in LDAP search filters, and caching all attributes returned in search results.
Sun Java System Directory Server: Sun Microsystems’ current directory service offering, found at [2].
OpenDS: An open source directory service implementation from scratch in Java, backed by Sun Microsystems and hosted at [3].
Banyan VINES: - The first scalable directory services offering.
IBM Tivoli Directory Server It is a customized build of an old release of OpenLDAP.
Siemens DirX Directory Server
Windows NT Directory Services (NTDS)
Critical Path Directory Server
OpenLDAP Derived from the original University of Michigan reference LDAP implementation (as are the Netscape/Red Hat/Fedora/Sun JSDS servers) but significantly evolved. It supports all current computer architectures, including Unix and Unix derivatives, Linux, Windows, z/OS, and a variety of embedded/realtime systems.
Isode Limited: High performance and high availability LDAP and X.500 servers.

There are also plenty of open-source tools to create directory services, including OpenLDAP and the Kerberos protocol, and Samba software which can act as a Windows Domain Controller with Kerberos and LDAP backends.