Royalty Free Music
ccMixter.org
ccMixter.org is a Creative Commons based sound resource. The site is comprised of samples, original tracks, and re-mixes. Since there are many flavors of the Creative Commons license, make sure you understand the license for the particular track you want to use.
Jamendo.com
Jamendo offers free music downloads from independent artists with the ability to filter your search results to music allowed in commercial productions. Just make sure you understand exactly what you need to do in order to use the music (link to the authors site, credit the musician etc.).