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 for Open Access Journal
Directory of Open Access Journals: General