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.

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.

Swiftype - The simplest way to add search to your website or application. Sign up for free.

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

Google Custom Search Engine - Google Custom Search is a platform provided by Google that allows web developers to feature specialized information in web searches, refine and categorize queries and create customized search engines, based on Google Search.