Replace your sluggish and rigid SQL search with the world open source leader Apache Solr wich powers the leading internet websites
WP-Elasticsearch replaces the default WordPress search with ElasticSearch.
WPSearch 2 is the missing site search for your WordPress installation. Install this plugin if you need a fast, relevant, google-like search.
wpSearchMu is a very powerful search plugin for WordPress Mu. It is based on the open source search engine "Lucene" which means its fast and relevant.