TCP Test Tool for Mac OS X
You might also find Wireshark useful.  I found a good video on Youtube to getting it setup.

http://www.youtube.com/watch?v=IxeHm0BKdwc

All the steps in the video may no longer be necessary as the changelog for version 1.6.0 says the following:


  Wireshark is now distributed as an installation package rather than a drag-installer on OS X. The installer adds a startup item that should make it easier to capture packets.