Power of RSS Mashups -- feeds let you send information, readers let you receive it. Pipes, PopFly, and GMashEd let you:filter,visualize, combine, correlate
ccMixter - a community music site featuring remixes licensed under Creative Commons, where you can listen to, sample, mash-up, or interact with music in whatever way you want.
Part 1 and Part 2 of a brief legal guide to sampling by Todd and Jeff Brabec of the American Society of Authors, Composers and Publishers
Public Domain Music - "a reference site to help identify public domain songs and public domain music"