Duplicate file detection.
Over time any computer will end up with multiple copies of the same file. After adopting Dropbox to backup files between multiple computers and share files with others, it became clear that more and more duplicates were accumulating in that system, which is wasteful of the space allocated.
To fix the problem I went with a simple open source solution: Doubles. Although the program doesn't have a fancy UI, it was enough to see the paths the duplicates lived on and remove those files. Combined with a filesize filter (to avoid being concerned about tiny files), it made quick work of my duplicates, reducing Dropbox storage by 10%.
One important feature of these kinds of tools is the ability to detect the same file by *contents* not by name. Many of the duplicates were simply renamed and relocated versions of the same file.