netcat - Netcat is a featured networking utility which reads and writes data across network connections...
UptimeRobot - Free Website Uptime Monitoring
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Uptime Kuma - A fancy self-hosted monitoring tool.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.