"Open Access to information – the free, immediate, online access to the results of scholarly research, and the right to use and re-use those results as you need – has the power to transform the way research and scientific inquiry are conducted. It has direct and widespread implications for academia, medicine, science, industry, and for society as a whole." --Open Access Week
Open Access (OA) does not mean free access. Some journals may charge nominal fees or impose an embargo period. Many publishers use a color system to distinguish between these different levels.
The OA movement is related to the open source movement (for software and coding) and Creative Commons. All of them help creators define how and when their content can be used and re-used.
Suber expanded on that article (plus other writings) into a textbook on open access:
Many of the items listed in the sites below are also indexed in the Directory of Open Access Books (DOAB) and the Directory of Open Access Journals (DOAJ).
These databases host some open access content and include them in their search results.
Most of this content is listed in the Directory of Open Access Books and the Directory of Open Access Journals.
Look for resources created under either a completely unrestricted use license, or a Creative Commons Zero (CC0) license, meaning that you can download, use, manipulate, share, distribute, or otherwise use them for whatever purpose you'd like without permission. It is strongly recommended that you credit the creator(s) of the image, but it is not necessary.