MarkTAW has a big list of his favourite ‘smallware’–little programs that do useful things. I had a look through the whole list, but the one that stands out is SpaceMonger:
This program shows you visually how much space is taken up by what folders/files on your hard drive. Think of it as a sort of square pie chart that allows you to zoom in to a folder. This is a very powerful program that allows you to literally see where all of your space is going. Not as popular as SequoiaView, but I think it’s better because SpaceMonger shows you directories and allows you to zoom in on them.
Wow, my music collection takes up a massive chunk–like 40%–of my hard drive. I also tried out SequoiaView, a similar product. It produces beautiful looking results, but I agree with Mark–they’re not nearly as useful. The file names and colour differentiation of SpaceMonger make it much easier to use.
DarrenBarefoot.com 
For us Mac OS X users, there’s something similar called “Disk Inventory X”
Free at http://www.derlien.com
Oops, that shoulda been http://www.derlien.com
(link works now)
I agree, SpaceMonger is a great program. I’ve used it before. There is an ongoing thread at the ArsTechnica forums called The neat application I stumbled across on the web. It’s about 35 pages long and contains links to great programs.
This comes standard with KDE, a linux window manager. It’s just another folder view option.
SpaceMonger is great… I saw a HUGE chunk of real estate taken up by one box, and I was able to right-click and delete it from withingthe program. VERY nice.