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.
mackerel.io - mackerel.io is a server and application performance monitoring platform.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
Network Server Suite - Network Server Suite is a network server monitoring software for Windows, Linux, or AIX that manage all of network servers, services, applications, and devices from a single, mobile-ready console.
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.