Iris looks like, but it runs 1000 dollars ... unelss you can find a pirated copy somewhere, of course.
Ethereal would have to be my choice as well since it tells you everything you could possibly want to know about each packet captured.
And to quote coder again, you can only sniff traffic on your local lan, assuming that you are connected to a hub and not a switch. Well, you can sniff traffic even if you're connected to a switch using ettercap.
and a side note, I use aimsniff to sniff AIM traffic