Dropbox Tip: Sync Files & Folders Outside Your Dropbox Directory

(If you don't use Dropbox yet, you should.)

The gist of it is to type this into Terminal:

ln -s /path/to/folder/name_desired_folder/ ~/Dropbox/desired-folder

or

ln -s ~/Dropbox/desired-file /path/to/folder/name_desired_file

You can also choose to put the original file or folder inside your Dropbox folder, and then create links in the other direction.

The information comes from here, and you can read about symbolic links in Unix here.