What is a good way in OS X of monitoring the speed of network data transfer?

Ping the router using Terminal.

Wire Shark is pretty sweet

Ping will only tell you about latency (specifically round-trip time or RTT) between two nodes. It doesn’t know anything about throughput.
Wireshark is a great packet-capture tool, and every serious network administrator should have it- but there’s a learning curve involved.
http://netalyzr.icsi.berkeley.edu/ offers an excellent tool to measure many aspects of your network connections. It runs in a browser (needs Java though) so it should work identically on all platforms.
The tests take a while to run, so be patient. I recommend this test to everyone (partly because Berkeley is using this test to survey the “health of the Internet”, which is something I’m concerned about.) Unfortunately this will test your bandwidth to their server, not to your local router.
A good command-line program for testing bandwidth is tcpblast. It works like this:

tcpblast -c(number of packets) [destination IP address]:[port]

where -c is the number of packets to send. 300 is default, but too short a sample IMO. Use 1024 or more. The test takes less than a second at that number on my network.

Here’s a sample: tcpblast -c 4096 102.168.0.1:80

Make sure you point tcpblast at an open port on your router. If you administer the router through a Web interface, port 80 should work.

Tcpblast probably is not installed to your Mac by default.

According to http://www.docum.org/docum.org/tests/setup/tg.php you can also use wget for this. Good to know…

ping wiil only tell if it is all okey with your connection. i don’t know any open source soft actually, but(it’s not a spam) my brother worked in company and they made a special programm for monitoring your activity. http://www.protemac.com/Meter/ – proteMac Meter.
it’s actually not spam)