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.
CodePen - A front end web development playground.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
JSFiddle - Test your JavaScript, CSS, HTML or CoffeeScript online with JSFiddle code editor.
Better Uptime - We call you when your website goes down
JS Bin - Sample of the bin: