Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
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.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
ResearchGate - Access scientific knowledge, and make your research visible
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
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.