Nipper!

Scapy is a powerful interactive packet manipulation program. It is able to forge or decode packets of a wide number of protocols, send them on the wire, capture them, match requests and replies, and much more. It can easily handle most classical tasks like scanning, tracerouting, probing, unit tests, attacks or network discovery (it can replace hping, 85% of nmap, arpspoof, arp-sk, arping, tcpdump, tethereal, p0f, etc.). It also performs very well at a lot of other specific tasks that most other tools can't handle, like sending invalid frames, injecting your own 802.11 frames, combining technics (VLAN hopping+ARP cache poisoning, VOIP decoding on WEP encrypted channel, ...), etc. »

packeth is GUI and CLI packet generator tool for ethernet. Primary it is developed and maintained for Linux, but some ports for Windows and MAC have also be done. It allows you to create and send any possible packet or sequence of packets on the ethernet link. Main advantages are that is really simple to use, powerful and supports many custom features. And lastly, it has the most beautiful web site of all the packets generators.

WPE Pro is a packet editor.Winsock Packet Editor (WPE) Pro is a packet sniffing/editing tool which is generally used to hack multiplayer games. WPE Pro allows modification of data at TCP level. Using WPE Pro one can select a running process from the memory and modify the data sent by it before it reaches the destination. It can record packets from specific processes, then analyze the information. You can setup filters to modify the packets or even send them when you want in different intervals. WPE Pro could also be a useful tool for testing thick client applications or web applications which use applets to establish socket connections on non http ports. »