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.
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.
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.
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.