I spent quite a while working with ADAM (Active Directory Application Mode) in Windows 2003 Server / XP on a previous project.  It's a great way to incorporate an LDAP directory into your application without requiring a full-blown AD installation, or domain memberships.  The Windows 2008 Server version has been renamed AD LDS (Lightweight Directory Services), which is a much better name.  LDS incorporates several improvements that make it even more attractive, including some very nice performance increases.   Bart De Smet has a very nice guide to installing LDS on 2008 Server and setting up a useful configuration.  Check it out if you've used ADAM and/or have any interested in LDAP.