FreeIPA!

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

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.

The enterprise-class open source LDAP server for Linux. It is hardened by real-world use, is full-featured, supports multi-master replication, and already handles many of the largest LDAP deployments in the world. The 389 Directory Server can be downloaded for free, and set up in less than an hour using the graphical console.

Keycloak is an open source identity and access management solution

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.