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X-Tunes is a free, fast and uninstrusive utility to control iTunes™ playback without bringing the iTunes™ window to the foreground. It requires Mac OS X 10.4 or later.

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.

While the X-Tunes window is visible on screen, you can also use some the keyboard to control iTunes™:
• use the left and right arrow keys to change the current track,
• use the up and down arrow keys to change the volume,
• use the Return key to pause and resume play.

You can read user reviews at the following web sites:

• VersionTracker.com
• MacUpdate.com
• Softpedia.com

X-Tunes was a highly-rated and popular utility in 2002-2003, and after being put on hold for several years, it's back in a universal binary version for Mac OS X Tiger & Leopard.
It's now a GPLv3 open-source project hosted on Google Code: you can download the latest version from the download area.