Cocoa Packet Analyzer (http://www.tastycocoabytes.com/cpa/) is a nice replacement for Wireshark on OS X if you don't want to have to use an X server.
I use tcpflow ( http://www.circlemud.org/~jelson/software/tcpflow/ ) It shows the entire stream, rather than just portions, like tcpdump. Great for debugging interactions between servers and clients, especially plain text protocols (like http!)