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 - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Dynamic Yield - Personalization & customer experience management
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Optimizely - A/B testing you'll actually use.
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.