Vertigo software has written a fantastic WPF reference application called Family.Show. You can run the app and download the source. It is a great sample:
It's realistic because there are shrink-wrapped applications for sale today that offer similar functionality;
It's applicable because it's a data-centric scenario, because the idea of a hierarchical tree is pervasive in many business scenarios (CRM and HR to name just two), it covers data visualization (also a key requirement for many modern business applications), and because it has a well thought out model that keeps data and presentation separate.
It covers many feature areas: data binding, styling and templates, animation, XML serialization, resources, rich text editing, image manipulation, Windows Vista integration and XPS output.
It's interesting: everyone has a family story to tell