“Samba 3.0 contains the first Open Source/Free Software implementation of Windows NT Primary and Backup Domain Controller functionality. Customers can transparently migrate their existing Windows NT domains to Samba 3.0 whilst keeping their existing user and group account databases. This enables significant cost of ownership savings over a Windows NT4 domain as a Samba 3.0 Domain Controller does not require client access licenses. Existing Windows tools can be used to manage a Samba PDC, allowing customer Windows expertise to be leveraged in a domain migration. A choice of LDAP back-ends allows integration with an existing customer directory service.” More here.