Quince is a neat idea and app from Infragistics that provides several ways to explore common patterns in UX design.
Quince is implemented in Silverlight, and does a nice job of taking advantage of the technology to enable a good-looking and an easy-to-navigate UI. You can explore patterns by specific user tasks, by relationship, and by viewing common patterns laid out on a typical UI wireframe.
If you’re looking for examples of common UI patterns, and why they’re used, to help guide you in your application design, Quince is a nice place to start. I know it’s going in my toolbox, as I can use all the guidance I can get when it comes to design.