FrameSeer

FrameSeer is an inexpensive network packet capture application for the Apple Macintosh. FrameSeer is a “universal binary” that runs on Mac OS X 10.4.5 (Tiger) or later.
Packet-capture applications go by a variety of names including, sniffing tools or sniffers (after the Network Associates "Sniffer" product), frame grabbers, packet monitors, and protocol analyzers.
Packet-capture tools are essential aids for diagnosing communications problems, investigating network security breaches, and for general network performance monitoring and troubleshooting tasks.
FrameSeer is a native Mac OS X ("Cocoa/Aqua") application and will help you to diagnose network faults with confidence.

Once you've used FrameSeer to track down your first network problem, you'll reach for it whenever your network hiccups.
FrameSeer can capture traffic on your Ethernet, AirPort, PPPoE (eg aDSL or cable-modem), PPP (eg dial-up modem) and loopback interfaces. FrameSeer can also open tcpdump files captured either on Mac OS X systems or on little-endian systems such as Linux.