Basically speaking, TrafficSpy is a Python re-implemenation of the Nethogs(see http://sourceforge.net/projects/nethogs/), I did some changes though :)
Unlike many traditional Unix/Linux network tools which group traffic by protocols or subnet, TrafficSpy can let you check your network traffic per process. You can easily find out which process consumes most of your bandwidth or check if there are any suspicious process connecting to your computer. Since it heavily relies on /proc, so it's a Linux specific tool.

todo: support ipv6/support multiple network interfaces.