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.

Platypus - Realtime server monitoring and reporting

Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.

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

Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.