Samba 4: Linux Active Directory Server
http://www.cyberciti.biz:80/tips/linux-active-directory-server.html

Linux does have directory server called OpenLDAP, but it requires good understanding and admin skills. MS-Ad has reputation for ease of use. Samba is a free software re-implementation of SMB/CIFS networking protocol mainly used by Microsoft. One of the goals of Samba version 4 is to implement an Active Directory compatible Domain Controller. Major features for Samba 4 already include:


  a.. support of the 'Active Directory' logon and administration protocols 
  b.. new 'full coverage' testsuites 
  c.. full NTFS semantics for sharing backends 
  d.. Internal LDAP server, with AD semantics 
  e.. Internal Kerberos server, including PAC support 
  f.. fully asynchronous internals 
  g.. flexible process models 
  h.. better scalablilty from micro to very large installations 
  i.. new RPC infrastructure (PIDL) 
  j.. flexible database architecture (LDB) 
  k.. embedded scripting language (ejs) 
  l.. generic security subsystem (GENSEC) 
  m.. over 50% auto-generated code!