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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PushMon - PushMon is a new kind of cloud-based monitoring system.
Better Uptime - We call you when your website goes down
Cronly - Keep track of your cron jobs and SSL certificates. Don't let them fail unnoticed.