The Subject index for the U.S. National Archives and Records Administration.
https://www.archives.gov/global-pages/subject-index.html
ALIC Archives Library Information Center - has over 99,000 bibliographic records.
https://www.archives.gov/research/alic/tools/online-public-access-catalog.html
arXiv – Cornell University Library - Open access to 1,466,632 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance, Statistics, Electrical Engineering and Systems Science, and Economics
https://arxiv.org/
ADS - Astrophysics Data System - three bibliographic databases containing more than 14.2 million records
http://www.adsabs.harvard.edu/
AULIMP: Air University Library's Index to Military Periodicals
http://fairchild-mil.libguides.com/az.php?t=14170 from which we also get:
BASE Bielefeld Academic Search Engine - world's most voluminous search engines especially for academic web resources - provides more than 120 million documents from more than 6,000 sources
https://www.base-search.net/ About 60% of indexed documents are free.
Citebase Search - an experimental, semi-autonomous citation index for the free, online research
https://eprints.soton.ac.uk/cgi/search/simple Primarily Mathematics, Computer science, & Physics.
Collection of Computer Science Bibliographies
https://liinwww.ira.uka.de/bibliography/index.html
Dimensions Vast data collection for search and analysis
https://covid-19.dimensions.ai
DOAJ Directory of Open Access Journals
https://doaj.org/
Google Scholar (has extensive case law references)
https://scholar.google.com/
Human-Computer Interaction Resources
http://hcibib.org/
Information Bridge: Department of Energy Scientific and Technical Information
Provides free public access to over 298,000 full-text electronic documents
https://www.osti.gov/
InfraGard Journal: Purpose of the InfraGard Journal is to advance the conversation on critical infrastructure protection by providing an academic forum to share ideas.
https://www.infragardnational.org/programs/infragard-national-journal/
Jurn Search - Jurn is a free-to-use online search tool for finding and downloading free full-text scholarly works.
https://cse.google.com/cse/publicurl?cx=017986067167581999535:rnewgrysmpe#gsc.tab=0
Microsoft Academic - Provides many innovative ways to explore scientific papers, conferences, journals, and authors
http://academic.microsoft.com/
My Science Work - Database includes more than 70 million scientific publications and 12 million patents.
https://www.mysciencework.com/
Naval Post Graduate - Scholarly content in all formats on subjects that support the current curriculum, instructional, and research needs of the Naval Postgraduate School.
https://library.nps.edu/
RePEc (Research Papers in Economics) is a collaborative effort of hundreds of volunteers in 99 countries to enhance the dissemination of research in Economics and related sciences.
http://repec.org/
SafetyLit Citations and abstracts of journal articles and reports from researchers working in the more than 35 distinct professional disciplines (architecture - zoology) relevant to preventing unintentional injuries, violence, and self-harm.
https://www.safetylit.org/
Sage Open Access Link to legacy open access articles.
https://online.sagepub.com/
Science Open Search engine for natural and physical sciences, social sciences, and humanities.
https://www.scienceopen.com/
Scientific Research Publishing Over 200 Open Access Journals
https://www.scirp.org/
Science.gov USA.gov for Science
https://www.science.gov
Semantic Scholar A project developed at the Allen Institute for Artificial Intelligence, more than 40 million papers from computer science and biomedicine.
https://semanticscholar.org/
United Nations iLibrary The comprehensive global search, discovery, and viewing source for digital content created by the United Nations.
https://www.un-ilibrary.org/?utm_source=UN.org%20academia%20page
WorldWideScience - a global science search engine (Academic databases and search engines) designed to accelerate scientific discovery and progress by accelerating the sharing of scientific knowledge.
https://worldwidescience.org/