Latest Update, WinDirStat (6/18/08)
Found a newer and better tool!  WinDirStat is also free and shows a nice graphical chart of the types of files and folders.
WinDirStat
Update for Windows Vista
I've been using Vista now for awhile and the tool below does not support Vista.  I've been using TreeSize Free which is working out great.  I've added it to my Send To folder so I can access it by right-clicking any folder.  Cheerio

This is one of the greatest little tools I've found lately!  http://foldersize.sourceforge.net/

It replaces the "Size" column in Windows Explorer, which normally doesn't show anything for folders, with a "Folder Size" column that displays the cumulative size of the folder.  This is incredibly useful for clearing HDD space.  This is definitely one of those apps you wish Windows included as a feature.