Even though "they" didn’t write this program; Microsoft’s Powertoys’ blog graciously mention this cool utility for showing the "treemap" of a hard drive’s contents.
SequoiaView — "Ever wondered why your hard disk is full? Or what directory is taking up most of the space? When using conventional disk browsing tools, such as Windows Explorer, these questions may be hard to answer. With SequoiaView however, they can be answered almost immediately. SequoiaView uses a visualization technique called cushion treemaps to provide you with a single picture of the entire contents of your hard drive. You can use it to locate those large files that you haven’t accessed in one year, or to quickly locate the largest picture files on your drive."