Ethereal is a NetXray-like tool that runs in an X-Window environment. I was most pleased to discover that it has already been successfully ported to OSX!
See http://www.ethereal.com (look under downloads - binary packages)
This was really good work by Peter F. Handel. If you have X-Widows running on OSX (you can do this with either the free XFree86 or Tenon's commercial XTools), this application is worth a look.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010605154430354