DirGraph provides a graphical view of the space used by your files and directories. It allows you to navigate around this view - zooming in to see greater detail and zooming out to see the bigger picture.
It also supports colour coding its display by file dates, so you can see at a glance how much of the space used is occupied by files that haven't been accessed for an age [and that should therefore have been committed to back-up storage long ago].
http://www.spillett.net/~dirgraph/