WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
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
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.