Google is a pretty powerful search tool. You knew that. You can use it to find information, but you can also use it to find downloadable MP3s, books, videos, and other items. We're going to assume you're just looking for legally available downloads, but the truth of the matter is if someone's posted an MP3 (copyrighted or not) to their web page, Google can find it.
But who has the time to memorize all the search modifiers like "-inurlhtm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(.mp3|.wma|.ogg) ?" That's where Google Hacks comes in. This handy little app will let you search for dozens of file types, song lyrics, cached pages, fonts, and other little goodies hanging out on the web.
Just download and install Google Hacks for Mac, Linux, or Windows, fire it up and enter your search term and check the boxes next to the type of search you want to perform. Your results will show up in your default web browser.
[spoiler]Available for users only