ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
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.
OpenSearchServer - OpenSearchServer is an open source application server allowing development of index-based...
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...
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.