I found this question on SF about network monitoring for OS X. One of answers described an open source solution called Zenoss.
From the looks of things it's very powerful. Here is a slightly old post about using it on OS X.
I downloaded and installed Zenoss. Unfortunately, it made my system feel unresponsive, even when the App was not running. (There are a bunch of process that run at startup, and eat up CPU).
Luckily, there is an uninstaller. I couldn't figure out how to get there in the Finder, but I just did:
open -a /usr/local/zenoss/uninstall.app/
It's probably no surprise that the uninstaller took more than 5 min to run, but it seems at least to have done its job and now everything is nice and snappy again.
