To get to the hidden folder on Windows, you'll need to set Windows Explorer to show hidden files in the folder options. Then type into the address bar:
On Linux, Killer Tech Tips says to use this path:
I found the folder on Mac by going to the Dropbox then using Finder's Find option. Click the drop-down box for the settings and selecting "Other..." then check "File invisible" and you can show only invisible items. The folder is called .dropbox.cache on my system.
Once you've found that folder, shut down Dropbox, delete the files and folders within the cache folder—but not the folder itself, and restart the service. You may want to delete only the oldest files, as suggested on Dropbox's forums. Whether you find the cache files taking up over 40GB of space, as some Dropbox users report, or not, you may be able to reclaim some valuable disk space.