tcpdump
Available OOTB - Out of the Box, in a Snow Leopard Server install.
Yesterday, I ran across a mysterious problem. All Google sites were blocked, inacessible from a browser window on the server itself. Google, Youtube, etal, none would load. Any other site on the WWW loaded fine.
The server was not under my direct control so I checked firewall, DNS, error logs, all the while wishing I had local access so I could install tcpdump. A short session with tcpdump and I could at least know for certain which end of the connection was doing the blocking.
Then today, a little over 24 hours later, I came across a blog post on entropy.ch that says tcpdump is installed by default on recent versions of OS X.
sudo tcpdump
It worked! Nothing to install. Just that simple. Will I ever learn? Just do what you think should work on an Apple machine. It may work exactly the way it should.