Konfabulator [via RAILhead design] is the coolest Mac OS X app I've seen in a long while. It's a manager for those tiny desktop widgets that everyone uses - calendar, battery monitor, weather - and these downloadable widgets look beautiful. Full use of the OS X transparency and shading. The app itself is polished to the extreme. Drag and drop installation, and the first time you run it it explains what it does and how to use it. All apps should be this easy.
Documentation is in the Widget Workshop (and it's comprehensive), or you can Show Package Contents of any widget to read the source. Their journal also gives you an idea about what the developers are thinking. This is very definitely one to watch.