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.
VictorOps - We make on-call suck less & help teams to solve problems faster.
UptimeRobot - Free Website Uptime 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.
xMatters - xMatters transforms event data into intelligent communication IT teams, avoiding outages and disruptions