arXiv: A free distribution service and open-access archive for scholarly articles in physics, mathematics, computer science, and related fields.
CORE: A multidisciplinary aggregator of open access research papers, providing access to millions of scholarly articles harvested from repositories and journals worldwide.
Google Scholar: A freely accessible web search engine that indexes the full text or metadata of scholarly literature across an array of publishing formats and disciplines.
Internet Archive Scholar: A free search engine that indexes academic papers across various disciplines, providing access to a wide range of scholarly articles, conference papers, and research materials.
CiteSeerX: An evolving scientific literature digital library and search engine focusing primarily on literature in computer and information science, offering access to a large collection of freely available research articles.
JSTOR Open Content: Offers a selection of open access ebooks, journals, and primary sources that are freely available to the public, spanning multiple disciplines and including historical and contemporary materials.
Social Science Research Network (SSRN): SSRN is a repository for scholarly research in the social sciences and humanities, including economics, law, and business. It provides access to working papers, preprints, and published articles.
IEEE Xplore: IEEE Xplore provides access to scientific and technical content published by the Institute of Electrical and Electronics Engineers (IEEE) and its publishing partners. It covers topics such as engineering, computer science, and technology.
ScienceDirect Open Access Journals: ScienceDirect offers a selection of open access journals covering various disciplines, including physical sciences, engineering, and social sciences.