WebGazer is an uptime monitoring platform that checks if your website is up as it should be and notifies you if anything goes wrong
PingPing - A simple tool to monitor your website and SSL
UptimeRobot - Free Website Uptime Monitoring
Flywheel - WordPress hosting and management built for designers and agencies.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
haveibeenexpired - No more expired SSL certificates
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.