X-Tunes is a free utility to to control iTunes without bringing iTunes to the foreground, or having the iTunes window on screen. Contrary to iTunes or other controllers, X-Tunes does not display a window permanently on screen, but only when you press a given key combination (Apple+Space by default). This window appears in front of all applications so you can click on its buttons while remaining in the current application. As soon as you release the key combination, the window disappears smoothly. More info and download link here.