I've just submitted an app to the Microsoft Win8 App Store, and its going through certification. It is a simple graphing program written in DirectX and Xaml. Expressions can be typed in using the textbox at the top, then the graph may be manipulated with intuitive gestures, including multitouch rotation and pinch zoom, and single touch or mouse arcball rotation.
This was created mostly to learn the basics of Flex and Bison, learn about DirectX and its interaction with Xaml, and learn about the store upload process. Depending on feedback, I have a backlog of features I'd like to add (implicit plotting, more intuitive interaction with the graph, etc.).