ANONYMOUS  best sniffer for mac. better than macsniffer, eavesdrop etc, ´cause simply it works on various interfaces!  
(Version 1.4.3)

KEVIN BALLARD  Very well-made app. Nice Cocoa interface, familiar tcpdump filters, very nifty traffic mapping feature, a pleasure to use. A shame it doesn't support all the features that more sophisticated programs like the free Ethereal (an X11 app, mind you, and with a much more complicated interface) does, such as display filters, protocol translation (such as decoding an HTTP conversation *as* HTTP and displaying each different part), etc. Also, Ethereal is free. So I'll stick with Ethereal for now, but I'll watch this app to see if it becomes useful enough to warrant purchasing.

Please note, I'm not advocating Ethereal over this for all users. I'm just saying that it's more powerful and free. However, it's an X11 app, it has a *much* more complicated interface, it has a much steeper learning curve, etc. If you haven't done any packet-sniffing before or you aren't interested in the advanced features of Ethereal, then this app is perfect for you.  
(Version 1.2.1)

ANONYMOUS  Excellent product. No bugs - well tested before release. May be the best Mac shareware yet for internet monitoring and packet sniffing. Although technically shareware, the demo version is crippled and you will get a highly annoying pop up window every minute or two reminding you to buy.  
(Version 1.1)