PagerDuty - Cloud based monitoring service
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.
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
UptimeRobot - Free Website Uptime Monitoring
Zenduty - Zenduty is a state of the art incident management solution that provides cross-channel alerts to your team whenever critical incidents occur.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring