"What do you mean there's insufficient space on my multi-terabyte disk?" If you've ever shouted that at your computer, first, switch to decaf; and second, download Folder Size. It's free.
MindGems's Folder Size "does what it says on the tin." It will scan your hard disk (it took about 2-3 minutes for it to finish my 1 TB "C" drive), and display the consumption of used space. Using an easy Explorer-style interface, it shows the space breakdown of the current folder as either a pie or bar chart in one pane, and a detailed listing by file or directory name in another. You might see that 60% of your disk space at the root directory is taken up by the "Users" folder, and then go into "Users" to find that 78% of that space is taken up by your "Games" folder.
Folder Size has many uses. It allows you to instantly notice if something is suspiciously large, which might indicate a directory filled up with junk data or undeleted files, or just let you realize how many files you've downloaded but never actually looked at. It can help you optimize disk usage by calling your attention to large collections of seldom-used data which could be easily moved to an external drive or archived offline. From a corporate perspective, it can show unusual patterns of disk usage on company machines. (There is no "enterprise" or network version; it must be run on each system separately.)
As compared to a similar free utility, SpaceSniffer, Folder Size provides fewer graphics but more technical details. One feature which Folder Size lacks, which I would have expected it to have, is the ability to double-click a slice of pie (or a bar in a bar chart) and shift to that folder. It's somewhat more cumbersome than it needs to be to shift between the visual display of the pie chart and the Explorer-like interface. When you hover the mouse over a chart, the selected item glows or jiggles, which certainly implies it's an "active" image you can click on or do something with, but it isn't.
Folder Size has one other minor issue. It can sometimes be slow in scrolling or refreshing, even though it's not constantly rescanning the disk. On the up side, Folder Size allows you to view, execute, or delete files from within itself, a feature SpaceSniffer lacks--this makes it superior for fixing space problems, as opposed to just finding them.
If forced to choose between these two freebies, I would give Folder Size the edge for its additional functionality and more detailed information. But both programs are free and consume very few resources, so there's little reason to have to pick.