• Directory of Open Access Journals – This site covers free, full text, quality controlled scientific & scholarly journals. They cover all subjects & languages.

• Hindawi - Hindawi is a rapidly growing academic publisher with more than 200 Open Access journals covering a wide range of academic disciplines.

• HighWire Press – host the largest repository of high impact, peer reviewed content with 1006 journals & 4,046,822 full text articles from over 130 scholarly publishers. HighWire hosted publishers have collectively made 1,559,277 articles free.

• ABC Chemistry – provides free full text journals in Chemistry.

• Bioline International - is a not-for-profit scholarly publishing cooperative committed to providing open access to quality research journals published in developing countries. BI's goal of reducing the South to North knowledge gap is crucial to a global understanding of health (tropical medicine, infectious diseases, epidemiology, emerging new diseases), biodiversity, the environment, conservation and international development.    

• BioMed Central – peer reviewed open-access articles in medicine and biology.

• BMJ – open access research articles on medicine.

• Find Articles – give you free access to millions of articles from thousands of top publications.

• Free Medical Journals – provides free medical literature, with 400 journals.   

• PubMed Central – a free archive of life science journals.

• Wiley Open Access – publishes authoritative peer-reviewed open access journals across many research disciplines.

• ERIC - Education Resources Information Center - is an online library of education research and information, sponsored by the Institute of Education Sciences (IES) of the U.S. Department of Education.

• arXiv 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..

•  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. Sixty percent of its content is open access, and you can filter your search accordingly.

• CORE- is a multidisciplinary aggregator of open access research. It allows users to search more than 66 million open access articles. While most of these link to the full-text article on the original publisher's site, five million records are hosted directly on CORE.

•  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. You can browse by document type, date, author, title, and more or search for keywords relevant to your topic.

• dblp Computer Science Bibliography- an online index of major computer science publications. Although it provides access to both free access articles and those behind a paywall, you can limit your search to only full-text articles. The site indexes more than three million publications, making it an invaluable resource in the world of computer science.

• Dryad- is a digital repository of curated, open access scientific research. It is run by a not-for-profit membership organization that aims to "promote a world where research data is openly available, integrated with the scholarly literature, and routinely reused to create knowledge." It is free to access, but note that there is a publishing charge associated if you wish to publish your data in Dryad.

• EconBiz- is a great resource for economic and business studies. A service of the Leibniz Information Centre for Economics, it offers access to full texts online, with the option of searching for open access material only.

• EThOS - Run by the British Library, it allows you to search over 400,000 doctoral theses in a variety of disciplines. Although some full texts are behind paywalls, you can limit your search to items available for immediate download, either directly through EThOS or through an institution's website.

•  OpenDOAR, or the Directory of Open Access Repositories- is a comprehensive resource for finding open access journals and articles. Using Google Custom Search, OpenDOAR combs through open access repositories around the world and returns relevant research in all disciplines.

•  Paperity boasts- being the "first multidisciplinary aggregator of open access journals and papers." Their focus is helping you avoid paywalls while connecting you to authoritative research.

• 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. According to the site, "all PLOS content is at the highest possible level of open access, meaning that scientific articles are immediately and freely available to anyone, anywhere."

•  Semantic Scholar- harnesses the power of artificial intelligence to efficiently sort through millions of science-related papers based on your search terms. According to the site, although some articles are behind paywalls, "the data [they] have for those articles is limited," so you can expect to receive mostly full-text results. Another feature is the extensive advanced search options, which allow you to search by cell type and brain region, among other things.

• ScienceOpen- offers open access to more than 28 million articles in all areas of science. 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.

•  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.

        There is not yet a specific option to search for only full-text articles, but because most of the papers on the site are free to access, it is not often that you encounter a paywall. You must become a member to use the services, but registration is free and enables you to interact with other scholars around the world.