Maestro is a simple interface to iTunes. It floats above all windows, is smaller than the "mini" mode of iTunes, and most importantly allows you to see and choose any of the playlists and/or songs in that playlist from iTunes.
The website is in french, but the application (made with AppleScript Studio) is in english too.
RequirementsMac OS X 10.2 or later, and iTunes.
*Previously available here