BleepingComputer Review:

KDirStat scans one or more hard drives and produces a detailed map or the disk making space hogs immediately apparent.

It does this by drawing a rectangle for each file, the relative sizes being decided by the relative sizes of the files. So, a very big file will be a very big box and a small file would be a small box.

KDirStat should work on any Unix-like system with a relatively recent X server.

Product Description from Stefan Hundhammer:

KDirStat is a graphical disk usage utility, very much like the Unix "du" command. In addition to that, it comes with some cleanup facilities to reclaim disk space.

While KDirStat is a KDE program, it runs fine on every X11 desktop, i.e., it runs on Linux, BSD, and lots of other Unix-type systems (Solaris, HP-UX, AIX, ...).