WebberZone Better Search - Better Search is a WordPress plugin that automatically replaces the default WordPress search with an enhanced contextual based search engine
Ajax Search Lite - WordPress Live Search plugin - Ajax search lite. A responsive live search engine for WordPress, for free.
Search & Filter - Search & Filter for WordPress allows you to build powerful search experiences for your users and customers - help them find the content they are looking for by creating filters for custom fields, taxonomies and more.
Advanced Woo Search - Advanced Woo Search - WordPress ajax search plugin for WooCommerce store. Search in product title, description, excerpt, sku, tags, categories, attributes, custom fields, custom taxonomies.
Relevanssi - Relevanssi replaces the standard WordPress search with a better search engine, with lots of features and configurable options.
WP Rocket - WP Rocket offers a caching plugin for Wordpress.