Relevanssi plugin replaces the standard WordPress search with a better search engine, with lots of features and configurable options.
Search results sorted in the order of relevance, not by date.
Fuzzy matching: match partial words, if complete words don’t match.
Find documents matching either just one search term (OR query) or require all words to appear (AND query).
Search for phrases with quotes, for example “search phrase”.
Create custom excerpts that show where the hit was made, with the search terms highlighted.
Highlight search terms in the documents when user clicks through search results.
Search comments, tags, categories and custom fields.
Adjust the weighting for titles, tags and comments.
Log queries, show most popular queries and recent queries with no hits.
Restrict searches to categories and tags using a hidden variable or plugin settings.
Index custom post types and custom taxonomies.
Index the contents of shortcodes.
Google-style “Did you mean?” suggestions based on successful user searches.
Download and info: http://wordpress.org/extend/plugins/relevanssi/