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:
Likewise, they've done that for TabGroup, Tab and StackPanel tasks:
There are other examples as well, virtually every major clickable element has a task pane associated with it. Very cool.
You can also drag and drop elements of your Ribbon around -- which really makes designing your control really easy.
Actipro has also made good use of the Property Browser, the collection editors have all been setup using the NewItemTypesAttribute which allows the end user of their controls to use the collection editors to configure the Ribbon:
Similarly, with non collection properties in the Property Browser:
Actipro uses the following Cider Extensibility points: DesignModeValueProvider (set a different value for the instance in the UI from that which is serialized to XAML), lots of Adorners, and Property Editing Extensibility.
For additional information, see http://www.actiprosoftware.com/Products/DotNet/WPF/Ribbon/DesignerSupport.aspx