The CSS Properties window is an add-in package to Visual Studio 2005. It does not change the behavior of any feature in Visual Studio 2005; it simply adds additional functionality specific to editing CSS in Design view.
Using the CSS Properties window, you can do the following:
Download it here!
Update: CSS Properties window works with Visual Studio 2005 Standard and above. The reason is that Express editions do not have extensibility DOM available, add-ins can only be added to the Tools menu.