Finding out what MySQL is doing

I was recently updating our production system when I noticed that our main MySQL database was going unusually hard. On a quad-core processor system, it was using up three processors simaltaneously. This prompted me to find out what MySQL is doing, and after being unable to read the logs, I did a Google search and ran across this useful little command :

show processlist

This runs a query (can be run from either the command line or the query browser) and shows you a summary of all the processes that MySQL is currently using.