WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
Curler - Dead Simple Free Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Zipy - Zipy is a debugging and prioritization platform that provides user session replay, frontend and network monitoring in one.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations