by Chris Gorvan on WordPress.org
Search Results Optimizer learns which results users find useful and automatically prioritizes them in future searches.

The search results stats page.
Change the way your WordPress website prioritizes search results, letting your users automatically train your website, teaching it which results are the most relevant for each search.
For each search you can set one result to be pinned to the top of the results list, so that regardless of result popularity or custom ordering, that result will be the first one displayed.
Choose a highlight colour and highlight the words users have searched for on the search results page. If a highlighted word does not appear in the excerpt it will attempt to create a better excerpt with the first searched word.
As well as ordering the first five results by popularity, you can choose to have results ordered by either:
Display an advanced search form on the search results page to let users filter results by post type, post categories and tags.
If the current search has zero results you can display links to searches that look similar, which other users have tried.