iostat is a command line I/O performance monitoring utility. It is present in almost every major Unix flavor in use today, and here you can get the version for Linux.
It works on both 2.4 & 2.6. What makes it different from other Linux utilities, which mostly show only I/O transfer rates, is that you finally can get important information about disk utilization, number of requests, average queue size and disk & queue wait times. No serious sysadmin should be without it. Check the screenshots to learn more.