Platypus - Realtime server monitoring and reporting
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
Healthy Servers - Making server monitoring easy
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Web Uptime - Multi-location website uptime monitoring