Mostly Creative Commons licensed music, including: Instrumental Music for Film & Video (“Find that perfect soundtrack or theme music for your film or video project.”) A lot of tracks, good search (or find) functionality. http://dig.ccmixter.org/
Citizen DJ (beta)
Make music using the free-to-use audio and video materials from the Library of Congress
Free Music Archive
This site offers both free (CC licensed) and royalty free (pay once for use right) music.