In spite of the double and triple digit gigabyte capacities modern hard disks, I still find myself running out of disk space. The big question is "where did the space go", and the hard part is finding which directories are big enough to be worth investigating.
For Windows, I use SpaceMonger. It produces a 2-D visual representation of which directories are using which space. You can see immediately which directories are the biggest, and you can drill into those to see what is in their subdirectories, etc. It's brilliant because it focusses your attention on the genuinely big directories.
For Linux, I've just discovered duv Visual Disk Usage. It's not quite as slick as SpaceMonger, but it basically gives the same information in more-or-less the same visual way. I just used it to liberate almost 3G of space on my hard disk.
SUSE Linux 10.1 comes with the Beagle search utility, and Beagle had created a ".beagle" folder that was over 1G in size! I nuked it. I also had a ".wapi" folder that was over 1G in size (I still don't know exactly what it was for, although it seems to be related to Mono or Novell's iFolder). Then there was a 155M ".thumbnails" directory that I really didn't need, and a number of other directories that were just old or unnecessary. Of course, don't delete anything unless you are confident you can live with the consequences.
It's amazing what you find out when you can actually see how your disk is being filled up.