For those who don't know scapy, it is a network tool for network tools. Instead of reinventing the wheel of writing a port scanner, a packet sniffer, traceroute etc.. scapy use them in a way you could find very surprising.

Scapy uses python as a Domain Specific Language. Python helps to organize grabbed data for latter use (scripting).

I wrote a simple recipe in Scapy's trac to help you writing custom packets from sniffed data. It shows how scapy can gather data from tcpdump and rewrite your own packet then.

If you want to go further than basic scapy functions Phil explained during Parsec 2005 you must read this.

For my daily use of scapy, when you are working with several dozens of machines where scripting is part of how to get things done fast, I see no other tool to accomplish my security work. And on top of that, scapy nicely integrates with graphviz and/or pdf output. Which is good for management or people who are not computer network addicts like me and want to see a human friendly representation.