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.
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
mackerel.io - mackerel.io is a server and application performance monitoring platform.
Observu - Observu offers website, server and API monitoring solutions.