Monitornator - Server monitoring made simple

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.

Nagios - Complete monitoring and alerting for servers, switches, applications, and services

Healthy Servers - Making server monitoring easy

Prometheus - An open-source systems monitoring and alerting toolkit.