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.
elicit - elicit is an on-site search software for internet, mobile devices and social media.
Scopus - Scopus is a bibliographic database containing abstracts and citations for academic journal articles.
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