The standard sidebar of the Finder!
The new sidebar that is part of the Finder in Leopard is entirely customizable, but what if you removed some of the defaults and you want to get them back. You could just drag them back, but there is an easier way. In the Finder preferences that I mentioned yesterday, you can check or uncheck the defaults of the sidebar in the "Sidebar" tab. It also lets you choose whether you want computers on your local network or on Back to my Mac to appear in the sidebar.