Klevu - Klevu offers instant site search solution for eCommerce stores.
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.
Dynamic Yield - Personalization & customer experience management
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Optimizely - A/B testing you'll actually use.
Sooqr - Sooqr Search provides relevant and innovative site search engine for the website.