iSuckMP3Blogs is a hastily-named Application (Written in Applescript Studio) that uses Jeff Veen's intelligently constructed wget command to suck down only the new MP3's from your favorite blogs. The mp3s all end up in a folder called 'SuckedMP3s' on your desktop.
Requires: OS X 10.3 or greater (Works with Tiger).
iSuckMP3Blogs is donationware. If you find it useful, you're encouraged to donate to further nerdery.