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