1. PubMed

https://pubmed.ncbi.nlm.nih.gov

PubMed® comprises more than 37 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.

2. Sci-Hub

https://www.sci-hub.st/

Sci-Hub is the most controversial project in modern science. The goal of Sci-Hub is to provide free and unrestricted access to all scientific knowledge. Sci-Hub started as a tool for providing quick access to articles from scientific journals - such articles are the main medium of communication of scientific knowledge today. Now Sci-Hub has grown a database of 88,343,822 research articles and books - freely accessible for anyone to read and download.