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.
Klevu - Klevu offers instant site search solution for eCommerce stores.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Swiftype Site Search - Swiftype Site Search helps to sell more, get the right answer to more people on the platform and surface relevant content for readers and followers.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Hawk Search - Site-search engine for eCommerce platforms.