The RealSearch WordPress Search Plugin replaces the default MySQL based search with a much more powerful Lucene based one, thus, delivering more accurate and relevant search results.
It is compatible with any kind of content, pages, custom post types, etc. It can even search in attached documents.
Requires PHP 5.3.0 (min), PHP 5.4.0 (all)
32 MB RAM (Depends on index size)
Search feature works from WordPress 3.8!