http://wiki.wireshark.org/Tools
http://bittwist.sourceforge.net/
Bit-Twist
Bit-Twist is a simple yet powerful libpcap-based Ethernet packet generator. It is designed to complement tcpdump, which by itself has done a great job at capturing network traffic.
With Bit-Twist, you can now regenerate your captured traffic onto a live network! Packets are generated from tcpdump trace file (.pcap file). Bit-Twist also comes with a comprehensive trace file editor to allow you to change the contents of a trace file.
Generally, packet generator is useful in simulating networking traffic or scenario, testing firewall, IDS, and IPS, and troubleshooting various network problems.