Export Artwork [OS X, found via Ben Hammersley] goes through all your music files, retrieves album artwork from Amazon, then embeds the artwork into the MP3 and/or saves it as a file in the album folder. The former option is ideal for displaying the artwork in iTunes (although Clutter does a good job of this too), while the latter is perfect for integration with WinAMP plugins (ShowCover for WinAMP 2, CoverTag for WinAMP 3).
A minor problem with Export Artwork is that, unlike Clutter, it doesn't display a 'buy this CD from Amazon' link, which is kind of a contractual obligation when using their data. It's one of those 'everyone has to play by the rules, even if they're badly designed' situations. Except it's not of course - if enough people break the rules, and nothing bad happens, then the rules get changed.
Speaking of Amazon, Amazon Hacks is available for pre-order (or maybe even sale) already. I don't know whether they decided to use blam! for the review formatting hack or not, but it should be a good read anyway.