Windows only: If iTunes is your music management tool of choice and you've found yourself wishing for a smaller interface you may want to check out SkiniTunes, a very compact player for your iTunes collection.
There are two interfaces within SkiniTunes. You can use the Mini Player which gives you playback controls plus album art and access to the star rating system.
Alternately you can use the Skini Player which is so narrow you can tuck it away against the edge of your screen or taskbar and easily forget it's there. The Skini Player is just tall enough for artist and song information with controls tucked to the side.
The Mini Player is skinnable as seen in the screenshot above, there are dozens of skins available here or you can use the templates available here to create your own. There is a free and a $US5 commercial version. The commercial version adds in a handful of features like a sleep timer, adjustable transparency, and the ability to assign global hotkeys to control iTunes from anywhere in Windows. SkiniTunes is Windows only and requires .NET Framework 2.0+. Thanks LauraBabarong!