Filelight allows you to understand exactly where your diskspace is being used by graphically representating your filesystem as a set of concentric segmented-rings. Filelight creates a complex, but data-rich graphical representation of the files and directories on your computer. The net-result is something similar to KDirStat, however the data is more dense, and the representation more informative. Most people tend to use Filelight to find out where their diskspace is concentrated, and this is what it is mostly designed to do.
And here’s a Flash-based alternative called torta: Link.