Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
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.
Klevu - Klevu offers instant site search solution for eCommerce stores.
ResearchGate - Access scientific knowledge, and make your research visible
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.