Platypus - Realtime server monitoring and reporting
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Monitornator - Server monitoring made simple
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
UptimeRobot - Free Website Uptime Monitoring