ccMixter

http://ccmixter.org/

Jamendo

http://www.jamendo.com/en/

SoundFlavor

http://www.soundflavor.com/

Find songs by genre, lyrics, decade, and free songs.

Free Play Music *One of my favorites*

http://freeplaymusic.com/

Free music and sound effects.

Incompetech

http://incompetech.com/m/c/royalty-free/

"Can I use this music in a Student Film? Commercial Film? Stage Production? Flash Animation? Instructional DVD? Relaxation CD? Slideshow?   Yes. Anything and everything - as long as I get a credit."

Partners in Rhyme

http://www.partnersinrhyme.com/

Royalty free music and free sound effects.

PodSafe Audio

http://www.podsafeaudio.com/

“We invite you to browse through the songs and artists on this site and download anything that strikes your fancy. The artists on this site have released their music under the Creative Commons license and agreed to allow their music to be used by you in any way and form for webcasting, as long as you don't take credit for the original work.” 

Sound Dogs

http://www.sounddogs.com/

The Free Sound Project

http://freesound.iua.upf.edu/

The Freesound Project is a collaborative database of Creative Commons licensed sounds. Freesound focusses only on sound, not songs.