How can I get DockPanel to make the last item fill to the bottom and right?

Answer 1

Dockpanel will automatically fill in the remaining space with the last item that does not have a DockPanel.Dock attached property specified and a Height/Width specified.

The sample I provided for you here illustrates this technique in the Shell.Xaml: http://stackoverflow.com/questions/1152282/looking-for-prism-example-of-modules-loading-themselves-into-a-menu/1154924#1154924

You can also find this fact in the MSDN documentation in the "Remarks" section for a DockPanel: DockPanel on MSDN