Creative Commons (description)

The Creative Commons has free, human-readable licenses for creative works. You can use these licenses to give very clear ideas of what people can do with your creative works, especially your music. If you release your music with no specific licenses, your music releases under an "All rights reserved" model, which might not suit your needs.

If you have never seen this site, explore it. It has a lot of very useful information, as well as guides to find other creative works that fall under these licenses that you can use in your own.