For .NET 1.x, there's really only the System.DirectoryServices namespace - not much more, unfortunately. That's your basic DirectoryEntry and DirectorySearcher classes, and that's about it.....
.NET 2.0 added the S.DS.ActiveDirectory namespace to handle structural objects like forests, domains, and so forth a lot easier. Check out this MSDN article New Ways to Manage Active Directory using the .NET Framework 2.0 on how to leverage those new capabilities.
For .NET 3.5 and up, there's a new S.DS.AccountManagement namespace which makes working with principals (users, groups, computers) a lot easier. Again, for an excellent intro, read this MSDN article Managing Directory Security Principals in the .NET Framework 3.5.
Hope that helps a bit!
Marc