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.
"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. "
Part of a book by Charles Petzold on programming Windows apps with WPF -- includes a link to all the code samples -- which are really great as a learning tool even if you don't have the book.
Localization of WPF apps is basically done via Resource files, but there's a neat app called "LocBAML.exe" which comes with the .Net SDK which can help with that...
Transitionals is a framework for building and using WPF transitions ... Wipe, Cut, Dissolve, Star, Blinds and 3D Rotating Cube are all examples of transitions supported by the Transitionals framework.
There are are a couple of valuable tricks already in this toolkit:
1) a fix for NUnit (and others) to allow unit testing things which must run in STA mode.
2) a way to make your XAML data binding testable by tracing data binding warnings