Now this is slick.  Thanks to Scott Munro where I first saw this.  I've often wanted to see the sizes of folders, but always seemed to be more of a hassle than it needed to be, but the FolderSizes tool (freeware from SourceForge) does just the trick.  You have to add a new column to your Windows Explorer view and it will display not only the file sizes but also the folder size for subfolders.  The calculations run in the background and I really didn't notice any slowdown as this background count runs.  In the past I've used the properties on the folder and/or other 3rd party tools to display folder size, now it's all right there in the explorer window.  Very slick.