FolderSizes  and Duplicate File Detective  are a pair of utilities that I’ve tried (not bought yet) that are very useful for helping to track your file server and see who is using your disk space and where you have duplicate files stored. They are both relatively inexpensive, the demos are very informative, and the tools are a lot more flexible than other free tools I’ve seen that do approximately the same things. Free is good, but these tools seem to go the extra mile if you have gobs of files to sort through and report on.
One of my favorite features when I tried FolderSizes is the ability to report on what files each Windows user owns. This was the reason I found the product in the first place, and I just haven’t had time yet to purchase and pursue my original goal.