Creating a Custom Markup Extension in WPF (and soon, Silverlight)

WPF currently, and Silverlight in v5, enables you to create your own custom markup extensions. Markup extensions are those little strings in XAML that are typically (but not always) enclosed in {curly braces} which, as the name implies, add new functionality to XAML. The most commonly used markup extensions are the {StaticResource}, {TemplateBinding}, and {Binding} extensions. Creating your own