Microsoft has made available a once only Premier Support utility that will automatically diagram your Active Directory enviornment. The “Microsoft Active Directory Topology Diagrammer” reads the AD configuration and then automatically generates a Visio diagram. It diagrams both AD and Exchange (2000+) topologies. It includes domains, sites, servers, admin groups, routing groups and connectors. If you have the full version of Visio you can manually makes changes to the diagram as well.

Get it directly from Microsoft downloads.