SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.