Prototyping has a wide range of applications. One is to gather user feedback in an early stage of the design process. A highly fidelity prototype might restrict the user in getting new ideas and inhibits honest feedback. In this case many use pen and paper for this early stage prototyping
But the artifacts you create are difficult to reuse. Wouldn't it be nice to get a "pen and paper" look and feel and easily use the result in XAML tools like Visual Studio and Expression Blend?
ProtoXaml is exactly that.
"Sketch" a User Interface to gather feedback and ideas and turn it by a mouse click into an artifact that Designer and Developers can refine. You can even switch back to the "pen and paper look"