http://freemusicarchive.org/ (must check licenses here)
http://www.jimmyg.us/ (free sign up to download)
freesound.org (free sign up to download) (sound effects & ambient tracks)
With Credit Given (Creative Commons) - Free!
By default, they are licensed under Creative Commons: By Attribution 3.0. To use this license, simply attribute the music in your piece (website, film, phone system, etc.) as is reasonable to the medium.