Tristan Pemble has posted some cool iTunes Music Store hacks, which include one tool that can grab the full size album artwork from any given album. All you have to do is find the album on iTMS, right click the title, choose Copy iTunes Music Store URL, and paste it into the field. It’s great for filling in missing artwork on ripped/downloaded music. Other cool hacks include a music video and movie trailer tool, and also one to view the raw XML response from iTunes’ servers (for debugging or general poking around).
Update: This method no longer works, but try my new PHP script here.