Platypus - Realtime server monitoring and reporting
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.
Monitornator - Server monitoring made simple
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Healthy Servers - Making server monitoring easy