This plugin replaces the standard WordPress search with an improved search engine with many functions and customizable parameters. You will get a better view of the results - your users will be grateful to you.

Info:

 KEY FEATURES
Search results are sorted in order of relevance, not by date.
Fuzzy Match: Match individual words if full words don't match.
Find documents matching a single search query (query OR) or require the display of all words (query AND).
Search for phrases with quotes, for example, "phrase search".
Create custom excerpts that show where the hit was made, with highlighted search terms.
Highlight the search terms in the documents when the user clicks on the search results.
Search for comments, tags, categories, and custom fields.
Multisites.
bbPress support.