Another great example of cool design time functionality in Cider (WPF Designer in Visual Studio 2008) comes from Actipro.  Their Office Ribbon control uses Cider Extensibility to add task panes invoked by an adorner that makes configuring their control a breeze.

For example, when the main Ribbon is selected a little widget adorner in the upper right hand corner is shown and when clicked will bring up a task pane that uses the Cider Editing Model to make updates to the XAML:

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Visual Studion VS, Office 2007, Cider, WPF