HttpWatch : An HTTP Viewer and HTTP Sniffer for IE and Firefox
Sitespeed.io : Sitespeed.io is an open source tool that helps you analyze and optimize your website speed and performance based on performance best practices. It col...
LibreNMS : A fully featured network monitoring system that provides a wealth of features and device support.
Buildbot : Buildbot is an open-source framework for automating software build, test, and release processes.
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.