Replace your sluggish and rigid SQL search with the world open source leader Apache Solr wich powers the leading internet websites
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)