I've been using this free tool called "Folder Size" (download available here) for a while now. It is a very impressive piece of (free) software that scans the entirety of a selected drive and produces an empirical picture of the distribution of occupied drive space. I have found this to be to be invaluable when cleaning out my disk space as it allows me to see the effective size of folders as well as files. The ability to natively view the size of a folder in explorer has been missing from MS Windows since its inception. Originally, I stumbled upon this application while attempting to address that specific concern, which it does. Beyond displaying the size of folders, it also shows the relative sizes of all items at the same branch of the drive hierarchy in both a percent column in the analysis results and a graphical display below as well as detailed file and folder attributes. Some of the options for the data displayed are type of visualization and default size basis (think KB, MB, GB, etc.). I highly recommend this software if you are going through the "where did all my disk space go?!?!?" routine.