DockArt for iTunes is a plugin for iTunes that will replace the Dock’s iTunes icon with the album artwork of whatever song happens to be playing at the moment.
Place the .bundle file in Library/iTunes/iTunes Plug-ins, then restart iTunes. There will be a new DockArt entry in the Visualizer menu up top, but you don’t even need to select it. If the current song has no artwork, the regular iTunes menu will show. Download DockArt for iTunes now at TopMacFreeware.