"Bling is a C#-based library for easily programming images, animations, interactions, and visualizations on Microsoft's WPF/.NET. Bling is oriented towards design technologists, i.e., designers who sometimes program, to aid in the rapid prototyping of rich UI design ideas. "
Balsamiq Mockups intentionally uses hand-drawn UI elements, so that people don't get attached to "that pretty color gradient" or think that your mockup has actual code behind it and is "practically done".