Funky Remixes "music found on this website is funky and free... a sweet collection of upbeat hip-hop, rap/rock, electronic and funky styled remixes."http://funkyremixes.com/
Podsafe Audio "a location where musicians can upload music under the Creative Commons license for use in Podcasts, Mashups, Shoutcasts, Webcasts and every other kind of 'casting' that exists on the 'net." http://www.podsafeaudio.com/
http://en.wikipedia.org/wiki/Wikipedia:Sound/list has a list of Public Domain music files - free to use. They are downloadable on .ogg format and will need to be converted to mp3 for use in a projest.
http://www.classiccat.net/ This is a database with links to performances - some which have downloads though you may need to register to download.
This site http://www.kunstderfuge.com/ has MIDI files for download (these are not MP3) which will play in iTunes. You can then convert them to audio files in iTunes or Quicktime to use in a project.