WPF Designer Extensibility
You can create your own custom designer experiences for the WPF Designer for Visual Studio. The WPF Designer provides a framework and a public API which you can use to implement custom adorners, tools, property editors, and designers. For a collection of full sample solutions, see the WPF and Silverlight Designer Extensibility Samples site.
In Visual Studio 2010, you can now also create custom designer experiences for the Silverlight Designer. Although this extensibility documentation references the WPF Designer, in almost all cases, the same concepts, steps, and code can be used in the Silverlight Designer.