The Expression Blend and Design team has provided a number of custom ValueEditors in the property inspector to enhance the usability of their standard Behavior set.
As an added bonus, we’ve introduced some limited extensibility to allow third-party controls and behaviors to reuse a few of these.
Specifically, the ElementPicker, the StateNamePicker and the StoryboardPicker can all be reused by placing the CustomPropertyValueEditorAttribute on the property you wish to use the editor.