Hidden in your home directory (/Users/username/) is a directory named .dropbox where the application keeps a database of your account configuration (encrypted) and what should be synchronized to your local Dropbox folder. There is also a ~/.dropbox/cache directory which can get huge. Since Dropbox has yet to add cache management to the agent application, you will need to manually clear this out if you want to free up space on your hard drive.
Enter Dropbox Cache Cleaner. This simple Dashboard Widget lets you see how big your Dropbox cache is and then gives you the option to clear it all away.