WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
Monitority - Simple website monitoring service (sms/email alerts)
UptimeRobot - Free Website Uptime Monitoring
Suprvise - Know instantly when your sites are down.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PingPing - A simple tool to monitor your website and SSL
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.