GPMonitor: This utility is composed of a service and a GUI. The service gets installed on any and all of your servers and workstations. There is also an accompanying ADM file that lets you set the behavior of the service. What this thing does is, each time a policy is refreshed, either in the foreground or in the background, the service generates an RSoP XML report. That report is then sent up to a server share you specify via the ADM file. So, if you have this service on 1000 workstations, then 1000 files are sent up to the server share each time policy is refreshed on a node. Actually, this is also configurable via the ADM so you can have it only send the file up every nth time that policy is refreshed. Using the UI tool, you can examine the RSoP in the GPMC-familiar format.
WinPolicies: WinPolicies is a GUI GPO Troubleshooting utility. Given that its a Resource Kit tool, its pretty rough, and presumes some strong knowledge of GPOs.
Linkspeed: Linkspeed is not a GPO tool, but it is a command-line tool for measuring link speed between two machines, and it has an /DC option that will measure link speed between a client and its DC. This is useful for troubleshooting GPOs, since GPO processing changes when a slow link is detected
Group Policy Inventory: This tool is similar to GPMonitor, but there's no service to install. Its a GUI utility that lets you perform simultaneous WMI and RSoP queries from remote machines. You can feed it a list of machines from a text file or have it dump the contents of an AD container that you browse to. It doesn't return all available RSoP options or WMI objects, but focuses on a subset such as showing the applications installed via Group Policy, Folder Redirection in effect, the list of GPOs processed by a given computer or user, etc.
To prevent you from browsing to other sites :-), I have included a link to the Windows 2003 Resource Kit page here.