It's an all to familiar proplem. Your favorite drive (HDD, USB, etc.) has filled up and you need to make some space. So you go into explorer and pull up your drive. Explorer shows you a list of folders and now its up to you to figure out which one is taking up the most space. This means a lot of right clicking. Enter FolderSize from RoteBetaSoftware. What a great play on words.
Folder size gives you a graphical representation of the folder sizes. Take a look at my ruby installation. You'll quickly figure out that the 'lib' folder is far and away the largest folder in the directory. You can even click directly on one of the bars in the graph to drill down into sub folders. Its a neat little utility (333kb) that comes in handy from time to time.