Iostat

iostat is a tool which allows you to monitor I/O statistics, such as the amount of data transferred over file devices or the console.

Handy arguments:

iostat -t da - only show stats for Direct Access devices; ie hard drives and the like.
iostat -w 1 - repeat stat display once per second.
iostat -K - use Kilobytes as the display block size, rather than whatever unit is native to the device in question.

Note that the very first line of output refers to the average of all I/O activity on each device averaged over the entire time the system has been up, whereas succeeding lines refer to the activity between the time that line is written and the time the last line was written.

see also: netstat, fstat