Wireshark

According to the Wireshark website, Wireshark is the "world's foremost network protocol analyzer". It allows a user to capture and analyze network traffic. I have no facts to support this, but would say that it's one of the main programs that a security or forensics user would use.

One of the nice things about Wireshark is that it's free and open-source. Generally, open-source projects, when they gather steam, are done pretty well and kept up to date (look at Linux). Plus it has quite a bit of functionality of looking at various protocols and streams.