GrandPerspective is a FOSS tool for Mac OS X that provides a treemap visualization of file layout on a disk. I created the treemap below from an 80 GB disk on my G4 towermac, which has both Mac OS X files (left) and WinXP files (right); the latter being a copy from the disk of my recently deceased Sony laptop). It certainly gives new meaning to the term disk blocks.
It's quite striking to see the greater number of larger aggregations of files on the Mac side vs. the many smaller files on the XP side. I guess that's why we don't need to do "defragging" on macs. :-)