Via Bill Simser comes the UI Patterns and Techniques website, which is a collection of user interface-related widget ideas and approaches collected from existing applications and people's comments on the web.
The author sums up the site by saying:
There's nothing new here...Each of these patterns (which are more general) and techniques (more specific) are intended to help you solve design problems. They're common problems, and there's no point in reinventing the wheel every time you need, say, a sortable table -- plenty of folks have already done it, and learned how to do it well. Some of that knowledge is written up here, in an easily-digestible format...
The site groups together concepts and explains when to use them and how. There's no code on the site (as far as I can tell), but this site is a good starting point when formalising ideas on how your app should behave, more than just look.