You should be aware that WinDirStat is actually a port of KDirStatYou should be aware that WinDirStat is actually a port of KDirStat, which is a Linux/KDE program. So, if you are looking for a Linux alternative to WinDirStat, you certainly should take a look at KDirStat. It is already packaged in most distros, just install it.
Another alternative is FileLight, already cited by Michael Mrozek, and the Konqueror plugin fsview (you can run it standalone from the command-line).