The Subject index for the U.S. National Archives and Records Administration.
ALIC Archives Library Information Center - has over 99,000 bibliographic records.
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
ADS - Astrophysics Data System - three bibliographic databases containing more than 14.2 million records
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
DOAJ Directory of Open Access Journals
Google Scholar (has extensive case law references)
Human-Computer Interaction Resources
Information Bridge: Department of Energy Scientific and Technical Information
Provides free public access to over 298,000 full-text electronic documents
Jurn Search - Jurn is a free-to-use online search tool for finding and downloading free full-text scholarly works.
Microsoft Academic - Provides many innovative ways to explore scientific papers, conferences, journals, and authors
My Science Work - Database includes more than 70 million scientific publications and 12 million patents.
Naval Post Graduate - Scholarly content in all formats on subjects that support the current curriculum, instructional, and research needs of the Naval Postgraduate School.
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.
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.
Science Open Search engine for natural and physical sciences, social sciences, and humanities.
Semantic Scholar A project developed at the Allen Institute for Artificial Intelligence, more than 40 million papers from computer science and biomedicine.
United Nations iLibrary The comprehensive global search, discovery, and viewing source for digital content created by the United Nations.
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.