AppMock by Telerik

Telerik AppMock allows you to quickly and easily create prototypes of your Windows Store applications. Providing a rich set of Windows 8 Style tools and components, conveniently grouped in galleries and categories, you can simply drag and drop UI elements onto the design surface to arrange the layout and flow of your application. You can organize the prototypes in pages (called sheets) and projects and in the end you can run the projects to see all that in action, by navigating through the pages using preset links (hot-spots).