There´s a lot of free music on the WWW! But music can be a lot of different things.... like WAW and MIDI. Here are some places that offers free music:
Midifarm-One of the largest archives at all! Over 12000 different MIDI files to download for free! There is both Midis for MAC and Pc.
Scour-If you are looking for a specific MIDi file, then try this exellent search-engine.
Midiplanet-This is a danish site with a lot of cool MIDIs. Here is from No Doubt to Metallica.
Cartoon Waws-This is a very large site, that offers free WAWS from different cartoons! You can also learn how to make your own WAWS.
Wawhounds-A great place to find free WAW files. They´ve got a lot of different tunes!
Soundamerica-Here´s over 24000 WAW files!