List of free databases and search engines (most listed in Wikipedia):
AGRIS: Agriculture Agriculture Food and Agricultural Organization http://agris.fao.org/ of the United Nations
Analytical Sciences Analytical National Science Digital Library and Digital Library Chemistry the Analytical Chemistry Division of http://home.asdlib.org/ the American Chemical Society
Anthropological Index Online Anthropology Royal Anthropological Institute https://aio.therai.org.uk/
arXiv Physics, Cornell University https://arxiv.org/ Mathematics, Computer science, Nonlinear science, Quantitative
Astrophysics Data Astrophysics, Harvard University System Geophysics, http://adswww.harvard.edu/ Physics
BASE: Bielefeld Multidisciplinary Bielfed University https://www.base-search.net/about/en/
Citebase Search Mathematics, University of Southampton http://eprints.soton.ac.uk/cgi/search/advanced Computer science, Physics
CiteSeerX Computer science, Pennsylvania State http://citeseerx.ist.psu.edu/index Statistics, University Mathematics
Collection of Computer science Alf-Christian Achilles Computer Science Bibliographies http://liinwww.ira.uka.de/bibliography/ Directory of Open Journals Lund University Access Journals https://doaj.org/
EconBiz Economics ZBW – German National Library http://www.zbw.eu/en/ of Economics – Leibniz Information Centre for Economics
ERIC: Educational Education U.S. Department of Resource Information Education Center https://eric.ed.gov/
GENESIS Women’s history London Metropolitan University http://www.lse.ac.uk/library/collections/featuredCollections/womensLibraryLSE.aspx
Google Scholar Multidisciplinary Google https://scholar.google.com/
PubMed Medicine U.S. National Institutes of http://www.ncbi.nlm.nih.gov/pmc/ Health’s National Library of Medicine
Jurn Multidisciplinary Jurn http://www.jurn.org/
Lingbuzz Linguistics Center for Advanced Study http://ling.auf.net/lingbuzz in Theoretical Linguistics, University of Tromso
Medline Plus Medicine U.S. National Library of https://medlineplus.gov/healthtopics.html Medicine, U.S. National Institutes of Health, and the U.S. Department of Health and Human Services
Microsoft Academic Search Multidisciplinary Microsoft https://academic.microsoft.com/
NBER: National Bureau Economics National Bureau of Economic of Economic Research Research http://www.nber.org/
PhilPapers Philosophy PhilPapers http://philpapers.org/
POPLINE Population, Knowledge for Health, Center http://www.popline.org/ Family Planning, for Communication Programs, Reproductive Health Johns Hopkins Bloomberg School of Public Health
Psychology’s Psychology York University Feminist Voices http://www.feministvoices.com/
PubChem Chemistry National Center for Biotechnology https://pubchem.ncbi.nlm.nih.gov/ Information and the U.S. National Library of Medicine
Science.gov Multidisciplinary Science.gov, U.S. Department of http://www.science.gov/index.html Energy, Office of Scientific and Technical Information
ScienceOpen Multidisciplinary ScienceOpen https://www.scienceopen.com/
SSRN: Social Science Social science Social Science Electronic Research Network Publishing http://www.ssrn.com/en/
WorldWide Science Multidisciplinary WorldWideScience Alliance, http://worldwidescience.org/ Incl. U.S. Department of Energy