Creative Commons Music Resources
Examples from SitePoint article
"Free audio clips and songs essentially have a limitless amount of uses from playing in the background of videocasts, being the opening theme to a podcast, or they can even just be used as a punctuation when a user clicks on a link. Of anything out there, Creative Commons licensed music has the most potential uses for content creators and designers alike." 
Over 7,000 free audio files as of this writing, all well categorized and easy to navigate. It also mixes in some social networking allowing to you have firneds lists, run a blog and more.
ccMixter offers up thousands of Creative Commons licensed remixed songs in just about every genre available. Make sure to read each tracks info page to see just which version of the license it follows.
Free-Loops offers up thousands of sound effects, vocal loops, drum loops and a whole lot more and all for, just as the name implies, free.
There is no music at Freesound.org, just tens of thousands of various sound samples for use in sampling, all of them licensed under the Creative Commons Sampling Plus License.
Jamendo is a site filled with nothing but Creative Commons licensed albums that take advantage of all the various versions of the license. Before using any of them, make sure you understand which version the album is using.
[shut down in 2010, between April 2 and August 11, link goes to Archive.org]
Another site offering you the ability to mix different tracks as you like, but also offers up a nice selection of various sound effects. All items are CC licensed, but license varies by track.
SoundClick is a site for unsigned and signed bands alike to offer up their music for download. It does offer a mixture of free and paid downloads, so you may need to do some hunting around for the ones you can use.
A social networking site built around independent artists uploading their music under a Creative Commons 3.0 by-share alike license." (http://www.sitepoint.com/blogs/2009/04/30/30-creative-commons-sources/)
From Other Sources
OpSound is a library of sound samples, and full musical tracks, uploaded for others to remix under a CC-BY-SA (3.0 unported) license.
Free Music Archive
The FMA aims to be a collection of all recordings of music available online free of charge, a musical equivalent of Project Gutenberg. As a not-for-profit project running an archive of verbatim copies, they can host any musical recordings released under any CC license without permission, as well as recordings that are in the public domain. If its license does not invoke the ND (No Derivatives) clause, music from FMA can be sampled or remixed, although care needs to be taken to honour the obligations of BY (Attribution), NC (No Commercial use) and SA (Share-Alike) clauses.
FMA also host recordings that are still under ARR (All Rights Reserved) copyright, by permission of the copyright holders, marked with their own Download Only license, and this music can't be sampled legally without permission.