1. Google Scholar
https://scholar.google.com/
Google Scholar is the clear leader when it comes to academic search engines. It harnesses the power of Google's search algorithms applied to research and patents. However, not all publications are of the same quality.
2. Microsoft Academic
https://academic.microsoft.com/
Microsoft Academic follows.
3. BASE
https://www.base-search.net/
BASE is overseen by the University of Bielefeld in Germany.
4. CORE
https://core.ac.uk/
CORE is an academic search engine that provides a link to the full text or full-text web page for every search result.
5. Science.gov
https://www.science.gov/
It's a fantastic resource as it aggregates and provides free access to research results from over 15 agencies. There's no need to query all these resources separately anymore!
6. Semantic Scholar
https://www.semanticscholar.org/
Semantic Scholar is an important giant in providing research results.
7. Baidu Scholar
https://xueshu.baidu.com/
Although Baidu Scholar's interface is in Chinese, the index contains research papers in both English and Chinese.
8. JURN
http://www.jurn.org
JURN is a free academic articles and books search tool. JURN is powered by a custom Google search engine (CSE) and has since expanded to cover university repositories of selected full texts and many additional e-journals in fields like science, biomedical, environment, economics, and law.
9. Directory of Open Access Journals (DOAJ)
http://www.doaj.org
DOAJ is a free search engine for scientific resources. It offers a vast array of subjects across multiple specialized fields of study. The directory targets students and researchers.
10. ScienceDirect
https://www.sciencedirect.com/
ScienceDirect is a gateway to millions of academic articles published by the scientific publisher Elsevier.