Develop a WinDirStat like program for Dropbox.com
Have the ability at a glance to see to see what files and folders take up the most space. Group them by folder and have a artistic/interactive interface to explore files.
Maybe go beyond and add other interesting data displays like a pie chart showing which file types take up the most space by percent. Bar-graphs showing amount of data added by month. Essentially, create a user dashboard for data lovers.