Wireshark is the premier network and protocol analysis tool. This is a must have for every networking engineer and IT professional. This tool will allow you to capture packets, perform analysis and then read the captures. Once you have narrowed down the packets you can dig deep and look at every detail of the packet in an intuitive interface.
This tool requires knowledge of networking and protocols to use at it's full potential, but newer networking professionals will want to use it as well as the premier tool for learning how networks work.