The ar Xiv e-Print Archive has been around since 1991 and is a well-known resource in the fields of mathematics and computer science.
It is run by Cornell University Library and now offers open access to more than one million e-prints.
Although you do need to register to view the full text of the articles, registration is free.
The advanced search function is highly detailed, allowing you to find precisely the research you're looking for.
Sixty percent of its content is open access, and you can filter your search accordingly.
Run by Indiana University, the Digital Library of the Commons (DLC) Repository is a multidisciplinary journal repository that allows users to check thousands of free and open access articles from around the world.The Social Science Research Network (SSRN) is a collection of papers from the social sciences community.The site offers more than 700,000 abstracts and more than 600,000 full-text papers.Public Library of Science (PLOS) is a big player in the world of open access science.Publishing seven open access journals, the nonprofit organization is committed to facilitating openness in academic research.It is, however, a highly useful research database for academics in a variety of disciplines.All the information is free to access, and it provides facts about every country in the world, including information about history, geography, transportation, and much more.The repositories it searches through are assessed and categorized by Open DOAR staff to ensure they meet quality standards.The Bielefeld Academic Search Engine (BASE) is operated by the Bielefeld University Library in Germany, and it offers more than 100 million documents from more than 4,000 sources.The Berlin- and Boston-based company was founded in 2013 with the goal to "facilitate open and public communications between academics and to allow ideas to be judged on their merit, regardless of where they come from." A multidisciplinary, community-curated directory, the Directory of Open Access Journals (DOAJ) gives researchers access to high-quality, peer-reviewed journals.It has archived more than two million articles from 9,519 journals, allowing you to either browse by subject or search by keyword.