SCI-HUB - It provides mass and public access to tens of millions of research papers
Internet Archive Scholar - Search engine for tens of millions of preserved research papers. This fulltext search index includes over 25 million research articles and other scholarly documents pres...
Unpaywall - Legally read research papers behind paywalls.
Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
Z-Library - The world's largest ebook library
Directory of Open Access Journals - DOAJ is a unique and extensive index of diverse open access journals from around the world, driven by a growing community, committed to ensuring quality content is freely available online for everyone.