Hulbee Enterprise Search - Hulbee Enterprise Search is another powerful site search solution that intelligently retrieves information and documents regardless of their source or location.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
SearchSpring - SearchSpring is an eCommerce site search tool.
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.
SearchBlox - SearchBlox is an out-of-the-box Enterprise Search Solution built on top of Apache Lucene/Solr.
Klevu - Klevu offers instant site search solution for eCommerce stores.