Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
PubMed.gov - PubMed comprises more than 29 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites.
SemanticScholar - An academic search engine that utilizes artificial intelligence methods to provide highly relevant results and novel tools to filter them with ease.
Web of Knowledge - Thomson Reuters Web of KnowledgeSM is a research platform that gives you access to objective...
Scinapse - Scinapse is a free, nonprofit, Academic search engine for papers, serviced by Pluto Network.
SCI-HUB - It provides mass and public access to tens of millions of research papers