The Internet contains a significant amount of incredibly valuable information which you may freely use in your web sites to benefit your web site visitors.
This content is covered under a wide variety of intellectual property laws. Some free content is placed entirely in the public domain, while other free content is released with a limited set of restrictions.
The current model for free control licensing is the Creative Commons Deed. The Commons Deed allows users to copy, distribute, display and perform the work. It also allows the work to be used in derivate works and to use the content for commercial purposes. The limitations are that the user must give attribution to the contributors and must not then distribute the work under a more restrictive license.
Two great sites which link to thousands of pages of free content available for your use are:
In addition, here is a useful Google Answer on finding copyright-free content.
For more information on the public domain and copyrights, visit the Center for the Public Domain.