Open access research is made freely available on the internet for reading and reuse. The degrees of openness vary but at the very least, this information - articles, conference papers, books, datasets, and more - is distributed through publisher websites and repositories without cost. For more on open access, see the library's research guide.

 General Search

BASE SearchThe Bielefeld Academic Search Engine, from Bielefeld University Library (Germany), searches though more than 71 million open access documents in more than 3,000 repositories across the globe.
HighWire SearchHighWire aids in "the digital dissemination of more than 3,000 journals, books, reference works, and proceedings," from Stanford University.
ROAD DirectoryDirectory of OA Scholarly Resources. An international collection of 11,000 OA journals, conference proceedings, and other scholarly works that have been assigned an ISSN (International Standard Serial Number) from UNESCO.
OpenAIREThe Open Access Infrastructure for Research in Europe project is an EU-based initiative to collect in one place "the entire Open Access scientific production of the European Research Area." They now provide access to more than 11.5 million publications from almost 6,000 repositories in EU member countries.

Articles & Journals

Directory of Open Access Journals (DOAJ)Indexes more than 10,000 journals from 136 countries. Journals in DOAJ are subjected to a rigorous application process to ensure that they maintain standards of peer-review and selectivity.
BioMed Central250+ and counting open access journals in the fields of biology, biomedicine, and medicine. Owned by Springer Science+Business Media.
E-Print Network (U.S. Department of Energy)"A gateway to over 35,300 websites and databases worldwide, containing over 5.5 million e-prints in basic and applied sciences, primarily in physics but also including subject areas such as chemistry, biology and life sciences, materials science, nuclear sciences and engineering, energy research, computer and information technologies, and other disciplines of interest to DOE."
PubMed Central3.3 million articles! "A free full-text archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM). ... PMC is a repository for journal literature deposited by participating publishers, as well as for author manuscripts that have been submitted in compliance with the Public Access Policy mandated by NIH and similar policies of other research funding agencies. PMC is not a publisher and does not publish journal articles itself."

 Ebooks & Textbooks

Directory of Open Access Books (DOAB)A service of OAPEN (OA Publishing in European Networks), DOAB is a discovery tool that finds books published elsewhere (as opposed to the OAPEN Library, which are books published with OAPEN). Currently listing more than 3,000 books, DOAB books are both free to read and free to share.
OAPEN (OA Publishing in European Networks) LibraryA collection of books published OA in Europe, available worldwide. Includes books published by Oxford University Press, Cambridge University Press, and Palgrave Macmillan, among many others. Books available through the OAPEN Library are free to read but not always free to share.
Digital Public Library of America BookshelfThe DPLA includes the digitized collections of several large universities and libraries throughout the country. The books here are more in the public domain (passed out of copyright protection) than open access but it is a valuable resource for historical texts.
National Center for Biotechnology Information (NCBI) BookshelfBooks and documents from the NCBI (part of the National Institutes of Health).
UMN Open Textbook LibraryA collection of open access textbooks, reviewed by professors, from University of Minnesota and a consortium of other institutions.
American Institute of Mathematics Open Textbook InitiativeThe American Institute of Mathematics has started an initiative to encourage the use of open textbooks. Here you'll find a list of approved titles and the standards by which books are judged.

 Open Access Repositories

Registry of Open Access Repositories (ROAR)ROAR sorts repositories by repository type, country, year founded, institutional association, and software, making it a useful resource not just as a list of repositories but as a way to study trends in repositories overall.
arXiv.org"Open access to 846,172 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics."
Social Science Research Network (SSRN)Almost 600,000 abstracts and almost 500,000 full-text papers in social science research.
Research Papers in Economics (RePEc)An economics-based repository that indexes publisher websites for more than 1.7 million documents in 82 countries. Volunteer-based, independently run.
Computing Research Repository (CoRR)Through "a partnership of ACM, the arXiv.org e-print archive, and NCSTRL (Networked Computer Science Technical Reference Library)," CoRR is a repository devoted to the rapid dissemination of articles in computing research.