DockArt is a nice little iTunes plugin that does something simple, and does it well. When installed it will change the iTunes icon in your dock to the album art of the currently playing track (assuming of course that it has album art). The developer is on the ball and recently released an updated version for iTunes 7.
DockArt is a neat little toy that'll offers a very unobtrusive visual reminder of what's playing. Best of all it's a free download from developer Greg Weston, who requests donations for Fidelco.