ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Hulbee Enterprise Search - Hulbee Enterprise Search is another powerful site search solution that intelligently retrieves information and documents regardless of their source or location.
Xapian - Xapian is an open source probabilistic information retrieval library, released under the GNU...
CommVault - Commvault is a site search platform that helps legal or compliance officers and corresponding end users to explore and retrieve files and email across different applications, storage devices and computers.
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.
SearchBlox - SearchBlox is an out-of-the-box Enterprise Search Solution built on top of Apache Lucene/Solr.