MacDropHide

MacDropHide has been discontinued as its functionality has been replaced by Broomstick.
MacDropHide is a simple app that painlessly allows Mac users to hide the Dropbox menubar icon.

MacDropHide works by taking Dropbox's icon files ("dropbox-box-icon-1.png", for instance), and renaming them to have ".hidden" on the end ("dropbox-box-icon-1.png.hidden", for instance). This means that when Dropbox goes looking for the icon files, it can't find them, and subsequently doesn't show anything in the menubar.