WooCommerce Instant Search + boosts conversion with the fastest search-as-you-type product suggestions, relevant results, and automated search filters
WPSOLR is the growing, blazing-fast, open source enterprise search plugin built on Apache Solr.
Powerful AJAX-based search alternative which supports faceting queries by taxonomies, terms, location, and post metadata.
Scriblio enables faceted searching and browsing of WordPress posts, pages, and custom post types.
Site search functionality for WordPress powered sites. The plugin is based on powerful hosted ElasticSearch engine (hosted for free)