This is htop, an interactive process viewer for Unix systems. It is a text-mode application (for console or X terminals) and requires ncurses.
IPTraf is a console-based network statistics utility for Linux. It gathers a variety of figures such as TCP connection packet and byte counts, interface statistics and activity indicators, TCP/UDP traffic breakdowns, and LAN station packet and byte counts.
Dstat is a versatile replacement for vmstat, iostat, netstat and ifstat. Dstat overcomes some of their limitations and adds some extra features, more counters and flexibility. Dstat is handy for monitoring systems during performance tuning tests, benchmarks or troubleshooting.
Dstat allows you to view all of your system resources in real-time, you can eg. compare disk utilization in combination with interrupts from your IDE controller, or compare the network bandwidth numbers directly with the disk throughput (in the same interval).
iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question "why is our ADSL link so slow?".
Glances is a cross-platform monitoring tool which aims to present a maximum of information in a minimum of space through a curses or Web based interface. It can adapt dynamically the displayed information depending on the user interface size.
Linux has always been able to show how much I/O was going on (the bi and bo columns of the vmstat 1 command). Iotop is a Python program with a top like UI used to show of behalf of which process is the I/O going on.
bmon is a monitoring and debugging tool to capture networking related statistics and prepare them visually in a human friendly way. It features various output methods including an interactive curses user interface and a programmable text output for scripting.