Advice on setting up a packet sniffer at home

Hi all -

I am a little confused about how to go about setting up a packet
sniffer on my main computer to monitor traffic my my other machines.
Specifically, I have two laptops - one running Windows and the other a
Mac running Mac OS X - that I would like to be able to see traffic and
packet information for. My main computer is connected via CAT 5 and
both of the laptops are connected wirelessly, all to my D-Link router.

What software/steps do I need to get/do in order to get this up and
running? Thanks in advance!

jf