iftop!

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.

iptraf-ng is a fork of original iptraf-3.0.0. IPTraf-ng is a console-based network monitoring utility. IPTraf-ng gathers data like TCP connection packet and byte counts, interface statistics and activity indicators, TCP/UDP traffic breakdowns, and LAN station packet and byte counts. IPTraf-ng features include an IP traffic monitor which shows TCP flag information, packet and byte counts, ICMP details, OSPF packet types, and oversized IP packet warnings; interface statistics showing IP, TCP, UDP, ICMP, non-IP and other IP packet counts, IP checksum errors, interface activity and packet size counts; a TCP and UDP service monitor showing counts of incoming and outgoing packets for common TCP and UDP application ports, a LAN statistics module that discovers active hosts and displays statistics about their activity; TCP, UDP and other protocol display filters so you can view just the traffic you want; logging; support for Ethernet, FDDI, ISDN, SLIP, PPP, and loopback interfaces; and utilization of the built-in raw socket interface of the Linux kernel, so it can be used on a wide variety of supported network cards. »