Windows Server Update Services 3.0

This is the number 1 download from TechNet.  WSUS 3.0 delivers new features that enable administrators to more easily manage and deploy updates across the organization. This package installs both the WSUS 3.0 Server and WSUS 3.0 Administration Console components, for all Windows Server 2003 SP1 supported languages. Additionally, the WSUS 3.0 client is included in all supported client platform languages. You must install the server components on a computer running Windows Server 2003 SP1 and later. You may install the Administration Console on a remote computer running Windows XP SP2, Windows Server 2003 SP1, or Windows Vista.  Get it here

Additional Resources
Release Notes for WSUS 3.0
Installing WSUS 3.0 on SBS 2003
WSUS TechCenter