When your hard drive fills up, the quickest way to find out which files are hogging the space is to use a specialist utility that displays your disk space usage visually.

There are a number of excellent free contenders that differ mainly in the way the disk space usage is portrayed:

SequoiaView [1] displays the space usage in a pretty manner using a technique called "Squarified treemaps", while the open source program WinDirStat [2] uses just ordinary rectangular treemaps not squarified.  It also offers a regular directory listing as well a summary chart showing usage by file type.

Now treemaps, squarified or otherwise, may be fine but until recently my favorite has been SpaceMonger [3]. It's visually cruder than the other two yet very effective and runs directly from the executable without the need for installation.  It's one of those programs that "just works."

But now there's a new contender called TreePie [4].  Basically, TreePie represents disk usage like a pizza with the size of each slice representing the space taken by each top level folder. Clicking a folder "slice" will then display a similar "pizza" breakdown of the sub-folders and files in that folder. It sounds complicated when I describe it but it's simple and intuitive in use.

It's also highly effective. Add to that the fact that it's also portable, has an alternative Windows explorer view and is free open source software and you have an easy "top product in class" recommendation.
My only complaint is its rather slow scan time. Still, this is not the kind of program you will be using everyday so a short delay is quite tolerable

[1] http://www.win.tue.nl/sequoiaview/  553KB
[2] http://windirstat.sourceforge.net/   640KB
[3] http://www.werkema.com/software/spacemonger.html   103KB
[4] http://treepieblog.blogspot.com/ 125KB including source code