We’ve published a set of set of add-ins which augments existing functionality in the Visual Studio 2005 Class Designer, Here is the link to the download site.
This only works with the Beta 2 build. It has some cool features like Pan/Zoom Window, MSDN Help on System Types, Dongles for easy creation of associations and inhertiance lines etc. You can find the list of all the cool features enabled by the add-ins here. There is also an add-in for the Distributed System Designer (available from the same download).
Note that the Powertoys have been moved from GotDotNet to the CodePlex site. Class Designer PowerToys for VS 2008 can be found at http://www.codeplex.com/modeling.