A free AI-based scholarly search engine containing over 10 million research articles in computer science and neuroscience. Brought to you by the non-profit Allen Institute for Artificial Intelligence (AI2) in Seattle, Washington.
Very good general databases that include a lot of scientific literature for all levels of researchers.
arXiv is a repository of electronic preprints in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv.
Project Euclid offers access to a growing platform of high-quality, peer-reviewed journals, monographs, and conference proceedings in the field of theoretical and applied mathematics and statistics. Table of Contents pages and article abstract pages are accessible to all users. Many publications have open access windows (a portion of their holdings are freely accessible), and a significant number are entirely Open Access.