www.Mendeley.com- a free reference manager and academic social network
www.Elsevier.com - provides information and analytics that help institutions and professionals progress science, advance healthcare and improve performance
http://www.sciencedirect.com - combines authoritative, full-text scientific, technical and health publications with smart, intuitive functionality so that users can stay informed in their fields.
www.Academia.edu - a platform for academics to share research papers
www.arxiv.org - Open access to 1,335,499 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance, Statistics, Electrical Engineering and Systems Science, and Economics
www.Sci-hub.cc - 62M research articles in available for free
https://cyberleninka.ru - russian scholar articles search portal
www.citeseerx.ist.psu.edu - CiteSeerˣ is a public search engine and digital library for scientific and academic papers, primarily in the fields of computer and information science
www.hal.archives-ouvertes.fr - HAL is an open archive where authors can deposit scholarly documents from all academic fields.
https://clarivate.com/products/web-of-science - Web Of Science is an online subscription-based scientific citation indexing service.
https://www.ncbi.nlm.nih.gov/pubmed - s a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics.
https://search.crossref.org/ - Search the metadata of journal articles, books, standards, datasets & more
https://clarivate.com/products/web-of-science/ - The Web of Science platform connects the Web of Science Core Collection to regional citation indexes