The netman suite was the first (AFAIK) publicly available free tool for watching network traffic. It consists of etherman (for Layer 2), interman (Layer 3) and packetman (a graphical packet sniffing tool). Unfortunately, the license prevented people from having access to the source code and now this tools is very hard to find and even then, the binaries are targeted for older versions of the platform OS.
The good news is that etherape strives to be everything that that the netman suite tried to be. With the very important exception that the etherape source is free and under the GPL.
iptraf is also very useful for environments without a graphical display.