Here’s a cool way of using Google to find MP3s. Essentially, it’s a simple matter of using a query like the following:
intitle:index.of "mp3" +"Keyword" -htm -html -php -asp "Last Modified"
(replace Keyword with the name of an artist, song, etc.)
There are a whole bunch of similar (and other) Google tricks on this website.
Update: The following two websites further automate this process: