CD Art Display is described as '(formerly Winamp CD Case) is a free desktop program that shows the current song cover in a skinnable display' and is an app. There are 2 alternatives to CD Art Display for Mac and Linux. The best CD Art Display alternative is TunesArt. It's not free, so if you're looking for a free alternative, you could try Covergloobus.
TunesArt offers new features for iTunes : display cover art and track information on your desktop (many themes available), get notification on track change, display bezel notifications on play/pause/next/prev, search your iTunes Library, scrobble to Last.
This is a simple application to show the cover and the info of what is playing in your favorite player. It also downloads the lyrics of the songs middle clicking on the applet.
