When I first saw Liquidx's Album Art Widget, I thought, "I don't want to have to jump to the Dashboard just to view album art." However, this widget does much more than that.
It does display the album art of the currently playing track, and will retrieve artwork for songs that don't have any (adding this found artwork to iTunes is easy). You can also change the ratings of songs, flip the widget over to view a list of other songs on the currently selected album and use it to control iTunes. Lastly, it will let you jump to the current song's home in iTunes or Amazon.