Samba is a free software licensed under GNU general public license and re-implementation of the SMB/CIFS networking protocol. Samba version 4 has been released. It is a major rewrite that enables Samba to be an Active Directory domain controller.
Samba 4.0.0 Available for Download
From the release note:
Samba 4.0 supports the server-side of the Active Directory logon environment used by Windows 2000 and later, so we can do full domain
join and domain logon operations with these clients.
Our Domain Controller (DC) implementation includes our own built-in LDAP server and Kerberos Key Distribution Center (KDC) as well as the Samba3-like logon services provided over CIFS. We correctly generate the infamous Kerberos PAC, and include it with the Kerberos tickets we issue.
When running an AD DC, you only need to run 'samba' (not smbd/nmbd/winbindd), as the required services are co-coordinated by this master binary. The tool to administer the Active Directory services is called 'samba-tool'.