I first wrote xdu in 1991 to display the output of the Unix "du" disk usage utility in graphical form. This made it much easier to figure out where all of the space was going in those days of tiny "always full" disk drives. It was included in the X11R4-X11R6 releases as contributed software. The above version 3.0 is the one from the R6 release. xdu has found its way into numerous Unix/Linux distributions.
Besides still being useful for "du" it has also found utility as a general purpose tree-data display program. E.g. I started using it in 1997 to display BGP routing data where AS paths were rewritten with slashes to look like directory trees. This let you quickly see how routes were distributed among your various peers. Others have reported using it for numerous similar things.
There's an enhanced decendent of xdu call xdiskusage by Bill Spitzak available on SourceForge.
P. Dykstra, firstname.lastname@example.org