MacSniffer

MacSniffer is a graphical user interface for the built-in "tcpdump". This is a packet sniffer that comes with all Macs, but has to be run from the Terminal app most likely.

By installing MacSniffer, you can use this with a graphical user interface that makes things easier. MacSniffer comes in a StuffIt package, so you have to have that installed it to get started. When you run the app, it will scan your ethernet by default. From the settings menu, you can change the interface to en1, which I take it is the Airport card.

When you start scanning, you will see how all the packets that are going around in your network start to appear on the screen. Each packet shows its timestamp (date + time), the source address and the destination address. Some packets didn't show an address, but you can check it by double clicking on the row, which opens a small pane at the bottom of the screen with information about the packet.

From the settings menu, you can choose how many packets to show on the main screen, the interface to be used and some header options.

All in all, MacSniffer is a free graphical interface for the built-in packet sniffer in your Mac. It works well enough for basic sniffing operations.