This is the de-facto standard network protocol analysis tool. It's designed for network experts who need to do deep network packet analysis.
Contains powerful filters and conversation views help to target relevant data.
Open-source, multi-platform, and best of all, free.
Room for Improvement:
Good working knowledge of TCP/IP is needed to use this tool, including packet structure, headers, and ports.
The volume of data on a typical TCP segment is so large that it can be challenging to capture and find the right data.
Can't sniff wireless networks without additional hardware, e.g. wireless dongle.
Somewhere out there, product managers are cursing Wireshark for providing such a great tool for free, when tools like this used to cost tens-of-thousands of dollars.