arXiv e-print Archive: Provides open access to over a million e-prints in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, and statistics.

National Academies of Science, Engineering and Medicine: has more than 200 books a year on a wide range of topics in science, engineering, and medicine, providing authoritative, independently-researched information on important matters in science and health policy.

CiteSeerX : CiteSeerX is a computer and information science database.

 

Open Access Library: OA library is a search engine with metadata for OA paper and also indexes journals. OA library currently has over 4,235,896 covering different academic fields.

Directory of Open Access Journals: General