PagerDuty - Cloud based monitoring service
Monitornator - Server monitoring made simple
VictorOps - We make on-call suck less & help teams to solve problems faster.
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
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.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.