SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.
NetMonster - NetMonster is a networking app by Michal Mrocek that comes up with features to enable you to get detailed information about your current network right on your mobile phone screen.
socat - socat is a relay for bidirectional data transfer between two independent data channels.