SpaceSniffer is a free and portable tool that shows you exactly where all your valuable drive space has gone.
The program uses a treemap approach that provides a very visual look at your current hard drive organisation: large files and folders take a lot of space on the screen, small ones consume hardly any at all.
If you don't understand why a particular folder is so large, though, you can just double-click it to zoom in and repeat the process. SpaceSniffer will redraw the display starting at that particular folder, and show you the largest files and folders within it. You can then right-click a file or folder to run or view it, or maybe just delete the object if you're sure it's unnecessary.
This alone would be very useful, but you can also customise the files that SpaceSniffer examines, filtering by file name, size or date. So you could, say, look at where all the JPGs are located on your hard drive, highlight every file that's been created in the last 48 hours, or perform any other analysis that you might require.
In addition, SpaceSniffer reacts to file system events so it's always in sync: if you're displaying a folder where files are being written, then you can watch the tree map change in real time. The program can also provide detailed reports on your hard drive use. And it's portable - just put SpaceSniffer on a USB flash drive and it's available for use anywhere.