Microsoft Active Directory Topology Diagrammer
http://www.microsoft.com/Downloads/details.aspx?familyid=CB42FC06-50C7-47ED-A65C-862661742764
Brief Description
The Microsoft Active Directory Topology Diagrammer reads an Active Directory configuration using ActiveX Data Objects (ADO), and then automatically generates a Visio diagram of your Active Directory and/or your Exchange 200x Server topology. The diagramms include domains, sites, servers, administrative groups, routing groups and connectors and can be changed manually in Visio if needed.