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.
Directory of Open Access Journals (DOAJ) – A multidisciplinary, community-curated directory, that 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.
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.