Sphinx Search - Sphinx is an open source full text search server, designed with performance, relevance (search quality), and integration simplicity in mind. Sphinx lets you either batch index and search data stored in files, an SQL database, NoSQL storage.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Apache Solr - Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and...
Butter.ai - Your team has Q’s, Butter.ai has A’s.
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.