Wireshark is an open-source packet analyzer that uses libpcap (*nix) or winpcap (Windows®) to capture packets and display them on its graphical front end, while also providing good filtering, grouping, and analysis capabilities. It lets users capture traffic at wire speed, or read from packet dumps and analyze details at microscopic levels. Wireshark supports almost every protocol, and has functionalities that filter based on packet type, source, destination, etc. It has the ability to analyze VoIP calls, plot IO graphs for all traffic from an interface, decrypt many protocols, export the output, and lots more.
Wireshark provides unlimited opportunities to study packets, which makes it a solid go-to for the network, system, and security admins.