Windows/Mac/Linux: Dropbox is getting ready to unveil selective file and folder syncing to the masses. If you just can't wait to try it, you can enjoy the new feature in their latest experimental build.
The feature still only allows you to selectively sync folders within the Dropbox folder, and not outside it. To try it out, go to the Advanced tab in your Dropbox Preferences to find the new Selective Syncing feature. There you can check or uncheck the folders and/or subfolders that you want to sync. Unchecking a folder tells Dropbox not to sync the folder, which will remove the folder on your local drive. It does not, however, remove the copy in your Dropbox account, letting you retrieve the files easily if you change your mind.
The new experimental version of Dropbox (which worked fine in all of our tests, but which you should download at your own risk) is free for Windows, Mac and Linux systems. Until then, if you're not ready to jump the gun on this new version, check out the cleverest ways to use Dropbox that you're not already using.
Dropbox 0.8.64 [Dropbox Forums]