iostat is command for monitoring devices (Hard Drives) attached to the system. This is another tool I feel a little foolish for just discovering.
iostat is a good tool to discover actual IOPS (Operations per second) http://en.wikipedia.org/wiki/IOPS 
I recently was tasked with making an RDS amazon server with a set IOPS on its hard drive. AWS offers guaranteed IOPS speed for a higher price. It’s a very useful feature to guarantee speed on your Database.
I wanted to test our current "Live" database to see what IOPS it is currently using. I found a few calculator sites out there to figure out our theoretical IOPS on the database server such as http://wintelguy.com/raidperf.pl . We got an estimate of ~400 IOPS. This number made me feel good, since the smallest IOPS you can buy on AWS is 1,000 and increment by 1,000.
This estimated rate is not the actual live rate, just an estimate of capacity. I wanted to see what the actual rate we were using is.
The iostat command is no longer installed by default on Ubuntu (my linux flavor of choice).
To install it run
> sudo apt-get install sysstat
Run the iostat command and read it
Looking at the notes made by Ben Mildren page 8 at http://www.percona.com/live/mysql-conference-2013/sites/default/files/slides/Monitoring-Linux-IO.pdf 
These stats show the statistics since start up.
The tps = IOPS. Since this machine was rebooted (many many months ago it has averaged 247 IOPS real usage)
What if I want to see more recent statistics?
Here is a command I ran
> iostat -dm 10 3
What this says is "-d" only display utilization report
"-m" says convert read/write to MiB
The next number, in this case 10, is the interval to look at. 10 is 10 seconds.
The final number, in this case 3, is the count number. Check 3 times.
So If I run this command I will see something like this
The first number displayed is the average since the server was last started. The following number is the average over 10 seconds.
If I leave off the last number and run
> iostat -dm 10
iostat will just continue to get the results every 10 seconds.
This is exactly what I was looking for and I am not going to go into any more detail on iostat. If you need more detail I would suggest first reading all of http://www.percona.com/live/mysql-conference-2013/sites/default/files/slides/Monitoring-Linux-IO.pdf 
 Wikipedia IOPS page
 Raid Performance Calculator
 Monitoring IO Performance using iostat & pt-diskstats