Packit is a network auditing tool. Its value is derived from
its ability to customize, inject, monitor, and manipulate IP
traffic. By allowing you to define (spoof) nearly all TCP, UDP,
ICMP, IP, ARP, RARP, and Ethernet header options, Packit can be
useful in testing firewalls, intrusion detection systems, port
scanning, simulating network traffic, and general TCP/IP
auditing. Packit is also an excellent tool for learning TCP/IP.

WWW: http://packit.sourceforge.net/