Generate a limited amount of random network traffic between 2 hosts

Go for simplicity, Try netcat.

If you want to crank out a limited amount, you can generate a file and pipe it into NC

nc 192.168.0.1 3333 > file.dat

Alternatively if you want to saturate your network you can set up a connection that will flood A's and B's between each other

on computer A

yes AAAAAAAAAAAAAAAAAAAAAA | nc -v -v -l -p 2222 > /dev/null

on another Computer (We'll call B)

yes BBBBBBBBBBBBBBBBBBBBBB | nc othermachine 2222 > /dev/null

Theres a plethora of references to things you can do with netcat, I highly encourage you to do a google sometime.

Use ping. You can script it to run random floods, and do all kinda of cool stuff. I'm not a MacOS expert but I'm pretty sure it is very similar to the Linux CLI options...

So to push the ports to the link speed limit run a flood ping (note must be root to do this, I'm running mine for 100,000 packets here):
$ping -f -c 100000 <destination IP>

Just wanted to say that Ostinato works both on Mac OS X and Windows.

On my 2.2GHz Core2Duo running Windows XP, Ostinato takes around 50% CPU. Speaking only for Ostinato - the reason it takes that much CPU even at low rates is to maintain an accurate (read constant inter packet delay) transmission rate in the presence of other contending applications.

You could also try tcpreplay

Disclosure: I'm the developer of Ostinato