OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
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.
SIGNL4 - SIGNL4 adds critical mobile alerting and anywhere incident management capabilities to your services and systems in no time. It bridges the ‘last mile’ from IT systems, machines and sensors to engineers, IT staff and workers ‘in the field’.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PagerDuty - Cloud based monitoring service
Better Uptime - We call you when your website goes down