A wide range of quality materials are available via Open Access, including:
Open Access does not mean 'inferior' in terms of credibility, editorial or peer review, polished formatting, or even sponsorship! And in some cases -- such as open textbooks that are continually refreshed and expanded on the Web - an Open Resource may be superior to traditionally-published works on the same subject.
Open Resources include:
To identify whether a given work is in the Public Domain or licensed via Creative Commons, look for a mark or statement indicating this status on the work itself. The metadata for the item may also contain a statement concerning its Public Domain or License status.
HOWEVER, do not assume that works containing no mark, license statement or copyright statement are 'Open Resources.' US Copyright law does not require that protected works carry a copyright notice, or that they be registered with the US Copyright Office. If you are unsure whether a work is being distributed as an Open Resource, consult your librarian or the author of the work itself.