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.
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.
mackerel.io - mackerel.io is a server and application performance monitoring platform.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services