No doubt, you have heard about FileLight. This is a KDE application that shows you how much disk space you use in a nice radial view.

And surely you are familiar with the Model/View classes in Qt 4. The separation of content and presentation can give you nice results. Once you start to build up a selection of models and of views, you can access and present you data with variety. A while ago I submitted a nice QAbstractItemView class, called WarpTree to the Qt programming contest. Now I've ported the Qt3 code of FileLight to the Qt 4 Model/View framework.

Have a look at the code.