"Where did all my space go?" This question has been asked by every computer user since the days of the 5.25" floppy (probably since the days of the ENIAC), and SpaceSniffer is a fast, free, easy, way to answer it. While SpaceSniffer in itself does nothing to reclaim space, it does provide you with a very good overview of what's occupying the nether reaches of your hard disk.
SpaceSniffer performs a quick scan of files and shows you your data in a series of nested boxes, each box being a folder or file. The size relationship is displayed graphically, so it's easy to see that your "Games" folder looms over your "Work" folder like Godzilla looms over Tokyo. You can zoom in on any folder to see its deeper composition. I found this an excellent tool to quickly find large chunks of data I didn't need, often in folders buried deeply enough that I do not stumble upon them casually. For example, I was in an alpha test for an MMORPG, now defunct, which took up some 4 gig of space I had completely forgotten about.
SpaceSniffer is a good, free, disk usage visualization tool. It can be very helpful for system administrators to get a quick overview of the contents of network drives and pinpoint unusual file or space allocations. Displaying data visually is more intuitive than simply looking at long lists of numbers--especially when simply seeing that a folder occupies five gigabytes does not inform you that, for example, four of those gigabytes are dedicated to a cancelled project. Given the non-existent cost (donations are requested) and the ease of use, I recommend this download.