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.
Platypus - Realtime server monitoring and reporting
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
Better Uptime - We call you when your website goes down
Nagios - Complete monitoring and alerting for servers, switches, applications, and services