Basically the Advantageous installer adds both an AppleScript to iTunes and a small application to your Mac. When you're browsing in the iTunes Store if you invoke the script from iTunes script menu in the menubar it will start the little application and search for the track in Amazon (via your default browser). It's not perfectly reliable (for instance, it didn't work with Choosy as my default browser and it messes up some searches), but it is a convenient way to surf for music in iTunes but take advantage of lower prices at Amazon. So now you can do to Apple what you've been doing to Barnes & Noble!
Advantageous mp3 is a free download from Robert Palmer.