iftop - iftop does for network usage what top(1) does for CPU usage.
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.
vnStat - vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network...
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
nload - Monitor network traffic and bandwidth usage in real time
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.