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.
"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. "
The Composite Client Application Guidance is intended to make it easier to design and build modular WPF and Silverlight client apps with rich user interfaces and loosely coupled, independently evolveable, UI pieces.
A sample project to implements a custom chrome with a UI similar to what Office 2007 does with its caption area... Basically achieves the effect, respecting the Theme chrome, and providing the same (not quite right) right-click menu on the caption as Office 2007 does
Asp.Net MVC is starting to look a bit like WPF ;-) aren't these DataTemplates?
"Templated Helpers - allow you to automatically associate edit and display elements with data types. For example, a date picker UI element can be automatically rendered every time data of type System.DateTime is used. This is similar to Field Templates in ASP.NET Dynamic Data."
Kaxaml is a lightweight XAML editor that gives you a "split view" so you can see both your XAML and your rendered content ... like XamlPad, but with intellisense, a tabbed ui, and more.
Blacklight is a UX focused code sharing project. Microsoft has released a bunch of technologies that allow designers and developers to work closely together to make beautiful software. This project is a collection of controls, samples, visual assets and ideas that has been put together by User Experience designers and developers to both show you what the technology is capable of (from a UX point of view), and give you code and samples that you can use in your own projects, completely free of charge (see the License tab above).