WordPress Sphinx Search Plugin allows to use Sphinx Search Server power to enable ultra-fast and feature-rich search on WordPress-based websites.
You can search by Relevance, Freshness or in both modes
Related searches widget is a great SEO tool for finding related search terms
Search Management Tools is a set of tools for managing search terms
Search Statistics Tool is an analytic tool which can help you analyze search trends of your blog
Widget settings have plenty of options to control widget behavior and quality of widget content