Finding what's causing I/O

[Francisco Reyes]

Looking at vmstat I see the "b" colun never hits zero and it's usually 
between 5 and 20.

Is there a way to find out which program(s) are causing the I/O?

In some of the machines it was near trivial to find the culprit, but have 
a handfull of machines that I am not sure what the cause of I/O is.

The machines are mailservers so they likely will not hold any particular 
file open long.