Active Directory
Introduction to Active Directory Objects (link).
Introduction to Active Directory Objects (link).
Good Active Directory / LDAP query tool is Softerra's LDAP Browser (link). Also see OpenLDAP.org.
.NET Interoperability
.NET Interoperability
Interacting with Active Directory with .NET code is done through System.DirectoryServices namespace. See page on MSDN (link, link).
Active Directory Federation Services
Active Directory Federation Services
Wikipedia page (link).
Windows Server 2008 R2 - Active Directory Federation Services page (link).
A Developer's Introduction (link) from 2006.