Swift Selection Search - Swiftly access your search engines in a popup panel when you select text in a webpage.
Yoast - Yoast offers plugins to improve SEO and optimize web sites and blogs.
Trufflepiggy - Quick Search - The all-in-one search addon: just select and search. Find on multiple websites at once. Search via overlay, context menu or omnibar. Featuring anonymous profiles. Easy setup: select from 1000+ user generated searches. Cross-browser sync.
Wordfence - Comprehensive security plugin for WordPress.
Selection Search - With this extension you can easily search for text you have selected.
Jackmail - A newsletter plugin for Wordpress.