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.
System Requirements

Supported Operating Systems: Longhorn (Windows Code Name) ; Windows 2000; Windows Server 2003; Windows Vista; Windows XP; Windows XP 64-bit
Microsoft .NET Framework Version 2.0
Microsoft Office Visio 2003 or 2007

Link: Download details: ADTD