netcat - Netcat is a featured networking utility which reads and writes data across network connections...
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.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Better Uptime - We call you when your website goes down