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
XAML Power Toys is a Visual Studio 2008 SP1 Add-In that empowers WPF & Silverlight developers working in the XAML editor. Its Line of Business form generation tools, Grid tools, DataGrid and ListView generation really shorten the XAML page layout time.
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.
Although it's nice to save the window position and size automatically ... it's frankly not worth an external library. However, this is a good sample of how to do attached properties, and if you have a general utility library that you include in all your WPF apps, this would deffinitely be a useful addition to it.
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...
WPF has two ways to create documents: XAML Flow documents paginate dynamically and flow into multiple columns at different zoom levels, etc. XPS documents are print ready documents on a fixed page size. Converting Flow documents to XPS isn't hard, even when you want to take advantage of the extra features like custom pagination, page size, margins, and headers or footers.