Planning and Implementing Federated Forests in Windows Server2003
Active directory collection
Active Directory Structure and Storage Technologies

Walkthrough: Adding Active Directory Objects
Introduction to Active Directory Objects
Active Directory Federation Services Overview  
Modifying Object Properties in an Active Directory Hierarchy

Introduction to Active Directory Objects

Querying Active Directory using .NET classes and LDAP queries
Active directory in C# Very interesting resource
How Do I...Access Active Directory?
Active Directory Application Mode With C#

User Management with Active Directory—Finding Users Including Windows 2003
Bulk Active Directory operations - an example  command line
Removing Incoming Email in MS Exchange, C# Example
Find an user in a multi-domain Active Directory enironment programmaticallyQuite interesting resource + Exchange

LDAP, IIS and WinNT Directory Services Very simple explanations of the basics...