I've always tried to keep close tabs on the files and programs that I've got on my computer (a product of my personality, perhaps), in part because hard drives tend to fill up eventually. I'm not as attentive as I used to be, with hard drive space being so cheap these days, but I've always looked for cool tools that help manage hard drive space, and have occasionally written my own rudimentary "disk usage" tools.

Thanks to Dotnet Boy's list of free utilities (inspired by Scott Henselman's list of Power Tools, which is also great), I found FolderSizes, a great free application that makes it easy to get a good overall picture of your hard drive usage, especially if you're looking for the biggest space-wasters. Also, thanks to Steve Hiner's comment, I've played with SequoiaView (also free), which gives an interesting graphical view of the entire contents of your hard drive. Cool stuff!

Update: FolderSizes is no longer free.