del.icio.us mp3s (list of popular mp3 links on del.icio.us; you can play them on the spot)
intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) Madonna hung up (Google search for unprotected directories of audio files or files of type mp3 and wma -- in this case Madonna's Hung up. Replace the artist and the song with anything you like.)
The Hype Machine is a web site that gathers songs posted on MP3 blogs and presents them in an easy to consume manner.
Using these links and many more you can discover great music, but not always legal mp3s.