Hmm, tcpflow is pretty good for being just a plain console app. I can't remember if you can get it from fink, or if you have to compile it yourself. If you use the "-c" option it spits data out to your terminal rather than logging to a file, and thus comes in quite handy if you need to quickly debug a network application.
If you need to do anything really heavy-duty, then just install ethereal via fink. Make sure you've got Apple's X11 installed first.