Wallice is a simple expense tracking app for Windows Phone 7. You can add and categorize your expenses to gain insight into your spending. You can compare different categories and how these change over time.
A friend and I built Wallice because we were curios of, at the time, new and shine Windows Phone SDK. As such it's not an app we continue to work on. Although the lessons we learned during the time has been handy ever since.
Technology and framework
Check the screenshots at the Windows Phone marketplace.