HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
Buildbot : Buildbot is an open-source framework for automating software build, test, and release processes.
LibreNMS : A fully featured network monitoring system that provides a wealth of features and device support.
SonoBus : SonoBus is an easy to use application for streaming high-quality, low-latency peer-to-peer audio between devices over the internet or a local network....
Wireshark is the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level. It is the de facto (and often de jure) standard across many industries and educational institutions.