X-Tunes for Mac

X-Tunes is a free, fast and uninstrusive utility to control iTunes playback without bringing the iTunes window to the foreground.

X-Tunes does not display a window permanently on screen, but only while you press a given key combination (Ctrl+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.