Packet Peeper is a network protocol analyzer (or 'packet sniffer') for Mac OS X. Its features include TCP stream reassembly, privilege separation, simultaneous capture sessions, filters, Python plugins and support for pcap capture files.
Love it, much lighter than Wireshark and as flexible as tcpdump! :)
This is really cool! Very similar to wireshark, but runs in the mac's native window manager. (Wireshark requires x11 on the mac the last time I checked.)