Wireshark is now available for OSX 10.5 (Leopard). You can download it from VersionTracker. There are still a few bugs in it, but it seems to work better than it did under 10.4 (Tiger).

Wireshark is the most popular packet capturing software available today. Wireshark is a continuation of development on the Ethereal product that I have used for years.
Wireshark on OS X and Linux enables you to capture data packets not only on your wired network interface, but also your WIRELESS! You have to pay money for this ability under Windows...nothing to do with Wireshark...just Windows.

Check out VersionTracker for the install package and Wireshark for more information on the program.

Happy sniffing.