I was looking to debug a problem involving clients connecting to FTP servers and was in need of a packet sniffer. A colleague of mine pointed me at an open source tool named "WireShark" (http://www.wireshark.org). This is a great tool. It's a bit like tcpdump, but it includes a beautiful GUI and does a great job at filtering packets and at exporting/importing data. It also run son Linux, Windows and Mac OS X.
Here's a screen shot: