ApacheDS is described as '™ 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'. There are eight alternatives to ApacheDS for a variety of platforms, including Linux, Windows, Mac, Self-Hosted solutions and CentOS. The best alternative is Univention Corporate Server, which is both free and Open Source. Other great apps like ApacheDS are FreeIPA (Free, Open Source), Microsoft Active Directory (Paid), OpenLDAP (Free, Open Source) and 389 Directory Server (Free, Open Source).
FreeIPA is an integrated security information management solution combining Linux (Fedora), 389 Directory Server, MIT Kerberos, NTP, DNS, Dogtag (Certificate System). It consists of a web interface and command-line administration tools.
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, and utilities, tools, and sample clients.
Enterprise-class open source Linux LDAP server. Hardened by real-world use, full-featured, supports multi-master replication. 389 DS can be downloaded for free and set up in less than an hour using the graphical console.
Go-lang LDAP Authentication (GLAuth) is a secure, easy-to-use, LDAP server w/ configurable backends. - Centrally manage accounts across your infrastructure - Centrally manage SSH keys, Linux accounts, and passwords for cloud servers.
OpenDJ is a directory server which implements a wide range of Lightweight Directory Access Protocol and related standards, including full compliance with LDAPv3 but also support for Directory Service Markup Language (DSMLv2).