"Bling is a C#-based library for easily programming images, animations, interactions, and visualizations on Microsoft's WPF/.NET. Bling is oriented towards design technologists, i.e., designers who sometimes program, to aid in the rapid prototyping of rich UI design ideas. "
WPF, in conjunction with the .NET Framework add-in model, allows you to address a wide variety of scenarios that require host applications to display UIs from add-ins.
Addins (plugins) for WPF can even do UI. I'm not convinced this is necessary to do UIs with plugins, but it's necessary if you want to allow untrusted code to do it.
get started with UI test automation using the new Microsoft UI Automation library, which is included in the .NET Framework 3.0 ... specifically designed for both accessibility and UI test automation tasks... test Win32 applications, .NET Windows Forms applications, and WPF applications.
TestApi is an experimental library of test and utility APIs that enables developers and testers to test WPF applications, Windows Forms applications, .NET Framework applications, and Win32 applications.