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.
Squadcast - Automate incident response, reduce downtime and enhance your tech teams’ delivery with a unified platform.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PagerDuty - Cloud based monitoring service
Better Uptime - We call you when your website goes down
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams