ApacheDS is described as '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' and is an app. There are more than 10 alternatives to ApacheDS for a variety of platforms, including Linux, Self-Hosted, Windows, Mac and Web-based apps. The best ApacheDS alternative is Univention Corporate Server (UCS), which is both free and Open Source. Other great apps like ApacheDS are Microsoft Active Directory, FreeIPA, 389 Directory Server and OpenLDAP.