Just a list of 20 (now 28) tools for the command line. Some are little-known, some are just too useful to miss, some are pure obscure -- I hope you find something useful that you weren't aware of yet! Use your operating system's package manager to install most of them. (Thanks for the tips, everybody!)
dstat & sar
iostat, vmstat, ifstat and much more in one.
htop & iotop
Process, memory and io monitoring.
Read more: Kristóf Kovács