Hunter TechLookup - Create lists of websites by technology for free
indeed - Find jobs using Indeed, the most comprehensive search engine for jobs.
Techmap.io - Techmap is a workplace search engine to find companies using technologies (e.g., Rails, PHP, Java, etc.) that are used internally. Finally, a "Google for Workplaces" that helps people find a place to work with technologies they love!
LinkedIn - LinkedIn is a business-oriented social networking service, mainly used for professional networking.
WhatRuns - Extension that helps you identify technologies used on any website at the click of a button.
Monster.com - Monster.com is one of the largest employment websites and job search engine in the world.