Note: This script hasn't been updated in years, and after the site redesign the script probably no longer works, at least not until a new update is released.
This small script adds a button next to each song entry under the free-music and artist sections of http://www.audiomicro.com. Using this alternative link you can simply right click it and save the .mp3 file it links to. It's the same file opened internally by the embedded player.
Simply install this script and it will do its job right after the page load its main contents. No special GM permissions are required so far.