Swift Selection Search - Swiftly access your search engines in a popup panel when you select text in a webpage.
Algolia - Algolia's Search API makes it easy to deliver a great search experience in your apps & websites. Algolia Search provides hosted full-text, numerical, faceted and geolocalized search.
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.
Google Custom Search - Google Custom Search enables you to create a search engine for your website, your blog, or a collection of websites.
Selection Search - With this extension you can easily search for text you have selected.
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.