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.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
UptimeRobot - Free Website Uptime Monitoring
Prometheus - An open-source systems monitoring and alerting toolkit.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Nagios - Complete monitoring and alerting for servers, switches, applications, and services