Musopen provides recordings, sheet music, and textbooks to the public for free, without copyright restrictions.
Internet Archive provides public domain and Creative Commons-licensed music.
Netlabels Free "virtual record labels from Archive.org
Jamendo.com provides Creative Commons-licensed music
IODAPromonet - The Independent Online Distribution Alliance represents independent music labels and provides some legal music in mp3 format for blogs and podcasts.
ccMixter - Another great source for Creative Commons-licensed music.
Free Music Archive Legal audio download site managed by freeform radio station WFMU