This application has nearly 300,000 downloads on VersionTracker alone, and many more on other sites, so it certainly must be one of the more widely used for retrieving album art. You control it using the Script menu in iTunes. Simply select one or more songs in iTunes, select Fetch Art from the script menu, and it searches Amazon.com for the album art. As with other add-ons, Fetch Art only works well if the music’s metadata is accurate, so use one of the add-ons mentioned above before you use it.
Free; Mac version only; fetchartblog.blogspot.com;