Creative Commons is a non-profit organization providing free tools that let authors, artists, and educators easily mark their creative work with the freedoms they want it to carry--from full copyright to public domain.

License examples:

Attribution.
You let others copy, distribute, display, and perform your copyrighted
work — and derivative works based upon it — but only if they give
credit the way you request.

Noncommercial.
You let others copy, distribute, display, and perform your work — and
derivative works based upon it — but for noncommercial purposes only.

No Derivative Works. You let others copy, distribute, display, and perform only verbatim copies of your work, not derivative works based upon it.

Share Alike. You allow others to distribute derivative works only under a license identical to the license that governs your work.