Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Algolia - Algolia's Search API makes it easy to deliver a great search experience in your apps & websites. Algolia Search provides hosted full-text, numerical, faceted and geolocalized search.
ResearchGate - Access scientific knowledge, and make your research visible
Klevu - Klevu offers instant site search solution for eCommerce stores.
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.