Like the physics preprint repository arXiv, SocArXiv collects preprints, but in the social sciences. Includes law & policy articles. A non-profit open access competitor to SSRN, which is now owned by Elsevier.
All fields of engineering, plus physics, electronics, control, and more. Helpful if you need technical papers. Hint: add the keyword 'review' to your search to see if there are literature reviews on your subject.
High impact journal articles in all fields of science; includes computer science. Includes citation tracking. Hint: select "review" under document types to get literature reviews. Can also search this with the Social Science Citation Index, linked above.