NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
Nubity - Nubity provides SaaS-based, centralised public and private cloud manager platform to manage and monitor servers.
mackerel.io - mackerel.io is a server and application performance monitoring platform.
The Galileo Suite - The Galileo Suite is an IT network and data management platform that offers solutions like operational intelligence, performance monitoring, data security, predictive analysis, etc.