Cocoa Packet Analyzer is a native Mac OS X implementation of a network protocol analyzer and packet sniffer. CPA supports the industry-standard PCAP packet capture format for reading, capturing and writing packet trace files.
 
Features:

• Packet capturing (NOT available for the Mac App Store version!).
• Analyze, display and filter packet trace files.
• Supports PCAP packet capture format.
• QuickLook plugin to get an overview already in Finder.
• Intel 64-bit CPU support.
• Printing of packets.
• Japanese and German localization.

Supported Types and Protocols:

• Ethertypes:ARP, IP (v4/v6), PPP, PPPoED/S, 802.1Q VLAN, MPLS
• Linktypes:Loopback, PPP, LinuxSLL, IEEE802.11-RadioTap
• IP-Protocols:IP (v4/v6), TCP, UDP, ICMP (v4/ v6), IGMP, MPLSinIP, L2TP, Mobility
• PPP-Protocols:IP, LCP, IPCP (v4/v6), CCP, PAP, CHAP
• PPPoE Discovery and Sessionstages
• Port based protocol detection: DHCPv6, L2TP, RADIUS, ESP

Requirements:
OS X 10.5+
Intel

Also available are:
Universal Binary 1.11 for OS X 10.5
Universal Binary 0.62 for OS X 10.4