PagerDuty - Cloud based monitoring service
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
VictorOps - We make on-call suck less & help teams to solve problems faster.
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
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.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.