Hunter TechLookup - Create lists of websites by technology for free
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
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!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
xZeitgeist - The most popular tweets categorized and ranked, every day
Better Uptime - We call you when your website goes down