Dead Man's Snitch - Monitor Cron Jobs and Windows Scheduled Tasks with ease.
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.
Vobla - Back up your Linux servers continuously
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Better Uptime - We call you when your website goes down