I've been looking for something like this for absolutely ages. I've tried everything I could find on versiontracker: Clutter, Album Cover Finder, Fetch Art; you name it, I've (probably) tried it. And the best I'd found before was the Artwork Search in Media Rage.

What made that better than the rest? The overview of several covers; and multiple stores to choose from. This latter is essential, as far as I'm concerned. I'm English and live in Germany, and have a lot of cds that just don't exist on Amazon.com. In Clutter you can check several stores (includine Amazon.de) but have to enter the preferences to do so; in FetchArt you actually have to restart the application to search in a different store. In Media Rage those stores are right above the preview window. Great. Only no Amazon.de.

Enter Export Artwork. This can be set to search any of 6 Amazon stores. I've never seen an app that search the French Amazon store - great for Les Negresses Vertes. It can even be set to search them all at once all at once!

And that's not the least of it. Export Artwork can also look through your entire library and make a playlist of only those songs which are missing artwork - then go through each piece and add artwork (similar to FetchArt, but much less traumatic). It has a very elegent HTML export function (which seems to be missing an index, but there has to be room for improvement). It will resize your artwork - and you can specify that it should only do so if the original is larger than the target size (great for home-made scans). And that's all without even talking about the export functions.

Buying advice: if you need all the tools which (the $30) Media Rage has on offer - and it has a truly impressive array - then that is your application of choice. For the pure and simple album artwork fanatic, it has to be Export Artwork.