StatusOk (Open source) - Get notified through Slack/email when your website is down
UptimeRobot - Free Website Uptime Monitoring
Better Uptime - We call you when your website goes down
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
Is it down by Freshping - Check if any website is down from 5 global locations
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.