When I'm organizing my hard disks or attempting to reclaim disk space, the Finder isn't always the most efficient tool. Instead, I turn to Tom Luhrs's DiskSurveyor to learn what's occupying my volumes. Drag a volume icon onto DiskSurveyor and, after quickly scanning the volume, it puts up a window where colored rectangles are arranged in columns to represent graphically the sizes of the files and folders on the volume. The height of the window represents the entire occupied portion of the volume. The first column shows the proportional sizes of all the top-level files and folders, the second column shows the proportional sizes of the second-level files and folders, and so forth. On my monitor, I can see about six columns at once (scrolling horizontally displays more).
Where there's room, an item's name is shown, and you can hover the mouse over any item to learn more about it. For a closer look at a folder, just click on it: this zooms the view so that folder occupies the whole first column. You can also see simple bar-charts or pie-charts of all volumes simultaneously, showing how much of each is occupied. Finally, you can export a window's contents as a text file, suitable for analysis with a spreadsheet or database program, or for searching with a text editor such as BBEdit, or for displaying graphically in DiskSurveyor at some later time.
When I first tried the program I thought it had a gorgeous, ingenious, and original interface, but I didn't imagine I'd have much practical use for it. A week or two later, though, it showed me instantly that the invisible Temporary Items folder had accumulated a lot of junk that wasn't being deleted, and a few days later it revealed that virtual memory had been turned on accidentally and was eating up the disk with its swap file. I instantly paid DiskSurveyor's shareware fee! My usual strategy is now to fire up DiskSurveyor from time to time, looking for blocks of color that seem disproportionately large; but I also like to use it just to roam around, getting a sense of what's where on my hard disks in the first place - DiskSurveyor is a great way to do this, because, unlike the Finder, it shows you several levels at once.
DiskSurveyor has almost no connection with the file system; Shift-clicking a folder's representation opens it in the Finder, but that's all (for example, from within DiskSurveyor you can't delete a Finder item or turn an invisible item visible). But I never feel this is a detriment, since there are other ways to accomplish these things; to implement them would probably detract from DiskSurveyor's purity, simplicity, and beauty.
DiskSurveyor 2.5 is $15 shareware; it requires System 7 or higher, and is a 450K download.