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soketi - Your simple, fast, and resilient open-source WS server.
UptimeRobot - Free Website Uptime Monitoring
websocketd - Turn any program that uses STDIN/STDOUT into a WebSocket server. Like inetd, but for WebSockets.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
StatusCake - Website Uptime Monitoring & Alerts โ Free Unlimited Downtime Monitoring
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations