WP User Manager - The most customizable profiles, community builder and membership WordPress plugin
WebberZone Better Search - Better Search is a WordPress plugin that automatically replaces the default WordPress search with an enhanced contextual based search engine
Jetpack - Jetpack offers SEO and security plugins for Wordpress.
Ajax Search Lite - WordPress Live Search plugin - Ajax search lite. A responsive live search engine for WordPress, for free.
UpStream Plugin - UpStream Plugin is a free-to-use project management plugin that allows you to manage various types of projects right from your WordPress site.
Relevanssi - Relevanssi replaces the standard WordPress search with a better search engine, with lots of features and configurable options.