The question should be interpreted from a general point of view and not targeted solely at web apps or desktop apps.
I have been looking around to find a simple and
Expression Blend (http://www.microsoft.com/expression/products/overview.aspx?key=blend) can be used to create quick mockups in XAML. You can store data for the mockup as inline XML in the XAML, or you can quickly convert it to WPF/Silverlight application and build basic business logic behind your mockup using Visual C# Express (http://www.microsoft.com/express/vcsharp/) or Visual Studio 2008.