Since 2005, The combination of OpenLDAP, Heimdal Kerberos, and Samba 3 has been a staple in the Linux Infrastructure, with other services such as FreeRadius, NFSv4, and AFS being tacked on for good measure.
Many if not most Linux based utilities support LDAP. Unlike Samba 3, which functioned as an OpenLDAP based application, Samba 4 completely replaces OpenLDAP, and Heimdal Kerberos. Consider the following. Samba 3, while far beyond what Windows NT4 was ever capable of, expanded the NT4 Domain concept far beyond it' design limiations. In the most recent era, Samba 3.5 and 3.6, created an enhanced form of NT Domain Authentication just for interoperability with Windows 7. (This is very fascinating because it uses Windows 2003 Sign and Seal with NT4 Authentication, something NT4 never could do.) So it can be be said, while Windows 7 expressly drops support for Windows NT4, Windows 7 has express support for Samba 3.
Yet the sword of Damoclese has swung over the head of Samba 3.x for a long while. Vista dropped support for NT4 Style System Policies, requiring administrators to resort to registry Trickery with Wine and third party policy tools such as NitroBit.
Samba 3 brought about a form of NT Domain that supported LDAP as a backend, could use Kerberos for Authentication both for file shares and joining the Domain. (Although only other Samba clients could utilize the Kerberos aspects of Samba 3.) Could delf out policy by OU. With help from OpenLDAP, Samba 3 could overcome the single PDC limitation, and all Samba Domain Controllers could be writable PDCs because OpenLDAP supported Multi-master Replication.
Beyond Samba, FreeRadius could use LDAP for authentication, Evolution could garner configuration information from OpenLDAP, for IMAP and SMTP settings (CalDAV Support was never added, even though there were feilds in the OpenLDAP schema for the three CalDAV based Calendar, Addressbook, and Task List.) This cooperated with eGroupware. Sudo could draw Sudoers from OpenLDAP, as could NSS. Each had their own unique Schemas.
Unlike when Windows moved from NT4 Domains too AD, the movement was simple, before, you had no Directory Service, and now, boom! you do. In the Linux world LDAP has been a reality for a long time. Many applications are built to participate in Open Directory based Domains based on OpenLDAP Schemas. What happens if the Schemas conflict definitions? How will this be resolved?