Everything you wanted to know about your hard drive and file system (and probably a lot more than you need to know) and more is what you get with TreeSize Professional ($50, 21-day free trial).
The tree in question here is of course the file and folder hierarchy, not the elm in your backyard. TreeSize Professional displays everything from wasted space to the largest files to the oldest files in any number of ways - including a 3D tree map, pie chart, and both vertical and horizontal bar charts. It also save stats in several different formats including Excel and XML.
TreeSize Professional also provides ready access to a number of Windows functions, such as the System Restore configuration dialog, Add/Remove Programs dialog, and others that you might find handy while you're cleaning up your system using the program. In the end, that's the program's most useful application - showing what's going on in your file system, helping you find files, then letting you move them, delete them, etc. One of the standout functions is a duplicate file search.
TreeSize Professional's interface strays a bit from the Windows norm (hint: Right-click and use standard keyboard shortcuts instead of looking for functions in the Edit menu) but it's otherwise easy to use. The program itself can be a bit bewildering at first, due the number of ways to view info. We do have one wish: The program is so good at finding files, we really wanted to use it for renaming them en masse.
There's a free (albeit less far less useful) version of TreeSize - TreeSize Free - that shows stats, but lacks charting and the pro version's search and manipulation functions.
TreeSize Professional is probably of most interest to IT types who want to profile PC usage on a box-by-box basis.