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

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.

Xapian - Xapian is an open source probabilistic information retrieval library, released under the GNU...

Site Search 360 - Site Search 360 enhances and improves your built-in CMS or product search with autocompletion, semantic search, filters, facets, detailed analytics, and a whole lot of customization options.

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

Relevanssi - Relevanssi replaces the standard WordPress search with a better search engine, with lots of features and configurable options.