A very impressive datagrid and date-picker for WPF available for free (in exchange for your name & email). Has a *really* slick "Card" view, as well as fully hierarchical standard grids and more. With all the usual in-place cell editors, XCEED's WPF DataGrid is fully styleable and templateable.
"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.