One of the best sources had been SingingFish (now known as AOL Audio Search). It is hard to find, but the latest AOL Toolbar still surfaces it:
But, what are the alternatives? Well, I found this article that highlights 10 other options that you may want to check out.
11 Best Places to Search Free Music- liquid parallax's Blog: "I wrote earlier about the death of Singingfish media search, an amazing resource of online music. Since then I’ve been seeking alternatives to singingfish.com and I’ve got 10 solutions that will help you find that free mp3 that got lost in the interweb. Starting in order from average to best (1 is average, 10 is best). I tested the searches using the technique I dub the “Paul Westerberg method” (the standard of not using today’s most well-known band, but not being too obscure either).
- search for a song/artist (or lyrics)
- find the MP3 you are looking for and "add to playlist" (or listen or download)
- build a playlist
- once you have done that you can download the playlist as a whole (as .m3u or .asx file)
- or, you can just choose to listen right from the site - this will launch a new browser window that leverages the flash-Winamp clone, Wimpy Player
I'm not sure where they are getting/creating their index, but they claim to have more that 2 million tracks and 700,000 lyrics cataloged. Check it out...