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
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PagerTree - On-Call. Simplified. PagerTree empowers teams to share oncall responsibility and respond faster when incidents occur.
Better Uptime - We call you when your website goes down