Alternatives to Active Directory

[Steve Meyers]

Has anyone successfully used anything (especially open source software) 
besides Active Directory as the primary login authentication for Windows 
computers?  I've seen pGina, but that is no longer maintained, and I 
don't think it works on Vista or Windows 7.  From what I've seen, Vista 
has a better authentication mechanism that should make it easier to use 
alternative directories, but I haven't been able to find a way to 
authenticate it against a generic LDAP and/or Kerberos source.

Thanks!

Steve Meyers