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.
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
UptimeRobot - Free Website Uptime Monitoring
Pingbreak - Pingbreak is an original and free monitoring service.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
DownDetector - A simple service to tell you whether a website is down.