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.

Apache Solr - Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and...

ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.

Coveo - Enterprise search technology for better customer support, customer self-service, and knowledge management in the digital workplace.

Swiftype Site Search - Swiftype Site Search helps to sell more, get the right answer to more people on the platform and surface relevant content for readers and followers.

Google Enterprise Search - Google Enterprise Search provides organizations with internal search capability and improved site search functionality.