A while ago I played around with Cairo and created a Python tool fring, similar to KDE's Filelight, however not interactive and very simple. Frédéric Back took my code and gave it a little GUI love, and this is the result:
Frédéric added a nice interactive GTK GUI and a fully asynchronous directory walker based on Gnome-VFS which runs in a background thread and thus doesn't block the UI. This makes the user interface snappier than Filelight's ever was. It's a lot of fun to navigate your directories like this!
The current version of fring is not yet polished for a public release. In the meantime, you can get the sources from the SVN:
svn checkout svn://svn.0pointer.de/fring/trunk fring
Yes, I am aware that a future version of Baobab will offer a similar view of the filesystem. However, it just was so much fun to hack on fring, and due to the power of Python it was so easy and quick to develop this tool, that we just couldn't resist to do it.