Most of the applications I’ve developed have used Active Directory in one capacity or another. .NET support for Active Directory in v1.0-1.1 pretty much sucks. Things are supposed to get better in 2.0, but for now these resources are invaluable:

Accessing Active Directory Through the .NET Framework by Robert Chartier – this is an excellent article on using the System.DirectoryServices namespace to manage users. Robert also creates a sample data access layer wrapper for modifying users. Nice!

Viewing Active Directory Objects via ASP.NET by Erick Sgarbi – Erick focuses on enumerating local users and binding the results to a DataGrid.

Querying Active Directory using .NET classes and LDAP queries by Sriram Chitturi – this article on CodeProject develops a nice Windows application that will allow you to test your LDAP queries.

Microsoft Visual Studio.NET Server Explorer Extensions for Active Directory – This server explorer add-in allows you to build Active Directory queries. After building queries, the add-in will create System.DirectoryServices.DirectorySearcher and System.DirectoryServices.DirectoryEntry objects. These objects are ready to be used as part of your application. 1373 KB

Using the .NET C# LDAP Library – Novell has some an LDAP library written in C# to go along with Mono. You can also download the LDAP libraries from Novell Forge and grab the Developing with the System.DirectoryServices Namespace API for Mono PowerPoint slide deck.