Queryly - Queryly Search Cloud helps readers find more relevant content instantly on the site.
Klevu - Klevu offers instant site search solution for eCommerce stores.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
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.
FACT-Finder Onsite Search - FACT-Finder Onsite Search algorithm delivers relevant results even when spelling errors occur.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.