Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
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).
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.