Open access (OA) refers to "free and unrestricted online availability" of literature, as defined by the Budapest Open Access Initiative.
Peter Suber lists the following qualities as characterizing open access literature:
Besides the Budapest Initiative, there are been two other influential statements that have helped shape the definition of open access:
Open access is not the same as public domain. Works in the public domain are not protected by copyright. Open access is compatible with copyright, and except for instances where OA works simply happen to be public domain as well, most OA works are protected by copyright. For instance, Project Gutenberg is an example of open access content where the books digitized are in the public domain because their copyright has expired. By contrast, the articles available through the Public Library of Science (PLoS) are open access, but still protected by copyright; PLoS applies Creative Commons licensing to the articles they publish to grant "unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited."
Creative Commons provides licenses and tools that help creators retain their copyright while authorizing others to make certain uses of their work, beyond what is provided by traditional copyright law. Creative Commons licenses are used by individuals and scholarly publishers alike; open access-model publishers, including the Public Library of Science (PLoS) and BioMed Central, utilize Creative Commons licenses as the legal implementation of the open access concept.
Get Creative! video by Creative Commons was originally found at: http://creativecommons.org/videos/get-creative
These are some sources of high quality open access content in the sciences and humanities.
The Educator's Guide to Producing New Media and Open Educational Resources by 
Open Standards and the Digital Age: history, ideology, and networks by 
