Active Directory

Introduction to Active Directory Objects (link).

Good Active Directory / LDAP query tool is Softerra's LDAP Browser (link). Also see OpenLDAP.org.

.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

Wikipedia page (link).

Windows Server 2008 R2 - Active Directory Federation Services page (link).

A Developer's Introduction (link) from 2006.