I use Dropbox to sync files across my Macs, but also to collaborate with others on several projects. One day, I woke up to find the disk space on my MacBook Air had dropped substantially. I started poking around, looking for large files, swap files and the other usual suspects. I eventually found a hidden folder inside my Dropbox folder: .dropbox.cache.
Looking inside this folder, I found that it contained three dated folders, for the past three days. (For example, 2012-03-01.) Inside this folder were a number of files, and since on one shared project we use a standard versioning system, I was able to see that there were subsequent versions of these files, with names indicating that they had been deleted.
All in all, I recovered 8 GB that day, because there were two projects where people had change a lot of files. On a desktop Mac, this won't usually make much of a difference, but if you have a MacBook Air, in particular, without much disk space, you may suddenly find yourself out of room. So, if you see a sudden decrease in disk space, you can check this folder. If you use Terminal you know how to get there; if not, you can open it from the Finder. Choose Go > Go to Folder, and enter ~/Dropbox/.dropbox.cache (assuming that your Dropbox folder is at the default location at the top level of your home folder; change the path if it is not). You can delete the folders with no worry of losing files.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120302003836825