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.
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.
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.
A1 Website Search Engine - Create search engines for online and offline website content.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Relevanssi - Relevanssi replaces the standard WordPress search with a better search engine, with lots of features and configurable options.