Windows only: SkiniTunes solves some of the deficiencies of iTunes' mini player, adding album covers, lyrics, instant ratings, and a few other nice features. The $US5 pro edition, however, locks down the global hotkeys.
That's kind of a bummer, considering it's one of the things iTunes users on Windows usually have to run one-trick scripts to enable. Still, SkiniPlayer is a pretty nice alternative to iTunes' somewhat under-powered mini player, and, as you might imagine, there are a number of slick-looking skins available for it. iTunes has to be running alongside it, but you can hide iTunes' main window and switch to an even smaller, edge-of-the-screen compact player. Right-click on SkiniPlayer and select More Options, and you'll get a glimpse at what the developers have hidden behind the $US5 pay wall. If you're a daily listener, that might not be too much to pay, but, then again, you might recreate some or all of SkiniPlayer's functions with one of many great add-ons.
SkiniPlayer's free version is for Windows systems only, requires iTunes. Thanks, LauraBabarong! SkiniTunes