Wireshark is a GUI version of tcpdump for windows.  Wireshark lets you look at what packets are going over your network.  It can also decode hundreds of different protocols and let you paw through what’s going on.  Very incredibly useful if you’re trying to debug why something’s not working.  This is a must-have for every network technician!

To use Wireshark effectively, you may need to find yourself an old ethernet hub that is not a switch.  An ethernet switch will hide traffic not destined for the computer with wireshark running on it.  What you need is an old hub that doesn’t do any sort of switching.  These sort of things seem to be getting harder to find these days.  If you have an old hub lying around, don’t through it away!