Wireshark

We've been using Wireshark to help debug and understand protocol flows. I've used Wireshark before, and its predecessor Ethereal, but I've been very impressed at how easy recent versions are to install and use (on Linux and MacOS, at least) for high-level software debugging.

The HTTP protocol decode is really useful, and it handles messy details like re-assembling TCP packets so that protocol units are clearly displayed.

Also, it works very well with a local loopback interface, so it's not necessary to fiogure out arcane filters to exclude background network traffic when debugging a local client/server interaction.

Under Linux, remember that the Pcap library is also needed - the Ubuntu package name is libcap2-bin. Under recent versions of Ubuntu, it is also necessary to set appropriate privileges: see http://wiki.wireshark.org/CaptureSetup/CapturePrivileges.