Creative Commons licenses are becoming a hassle-free way for musicians to publish their work using a standard template license usually allowing for some type of royalty-free use.

You can use the Creative Commons site to search for music which is available under such a license, but as there are different types of Creative Commons license, you will need to check the details of the specific license for any music that you find and want to use:

http://creativecommons.org/audio/

In addition to the "Find" facility on the above page, check out the Featured Audio Sites as this contains links to  many other sites where you may find royalty-free music.