Once content has been created in a tangible form, it is automatically copyrighted under US law. Content creators may choose to "open" their content in order to provide others with permission beforehand. If materials are listed as open, then they may be used freely without requesting permission or paying a fee. Many open access content has a license applied to it, which requires certain conditions in exchange for use of the content (ie. attribution, non-profit use).
Open content does not necessarily mean free content. The key point is that open materials are open for distribution.
Below is a list of legitimate, vetted depositories of open source documents ranging a wide variety of subjects.
A open-access archive for scholarly articles in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics. Materials on this site are not peer-reviewed by arXiv.
Complete digitized back runs of core scholarly journals.
Provides access to electronic journals issued by Japanese academic institutions available to the public at no cost.
Public Domain, Fair Use and Open Resources found online.