Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
Scopus - Scopus is a bibliographic database containing abstracts and citations for academic journal articles.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
SemanticScholar - An academic search engine that utilizes artificial intelligence methods to provide highly relevant results and novel tools to filter them with ease.
ResearchGate - Access scientific knowledge, and make your research visible
Synthical - AI-powered collaborative research environment. You can use it to get recommendations of articles based on reading history, simplify papers, find out what articles are trending, search articles by meaning (not just keywords), and so on.