NetHogs

NetHogs is a small 'net top' tool. Instead of breaking the traffic down per
protocol or per subnet, like most tools do, it groups bandwidth by process.
NetHogs does not rely on a special kernel module to be loaded. If there's
suddenly a lot of network traffic, you can fire up NetHogs and immediately see
which PID is causing this. This makes it easy to indentify programs that have
gone wild and are suddenly taking up your bandwidth.