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.