I've just been playing around with SequoiaView (which is a lot easier to use than it is to spell!). It's a cool utility which displays the contents of your hard drives using a visualization technique called Cushion Treemaps. It makes it very easy to figure out which files and directories are taking up your hard disk space, and it produces some glorious mondrian-esque artwork as a neat side effect!

Here's a piccy of my C:\ drive:


Free download is available at http://www.win.tue.nl/sequoiaview/