Packet Peeper is a free network protocol analyzer (or 'packet sniffer') for Mac OS X. Its features include:
The latest version of Packet Peeper (and source code) can be downloaded here.
Packet Peeper is Copyright 2006, 2007, 2008 Chris E. Holloway.
Packet Peeper is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version. Packet Peeper is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with Packet Peeper. If not, see http://www.gnu.org/licenses/.