UptimeRobot - Free Website Uptime Monitoring
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Uptime Kuma - A fancy self-hosted monitoring tool.
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.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.