On Windows I used to bring up the Task Manager (Ctrl-Shift-Esc) and quickly change the priority of a process, e.g. to a lower priority so that some process doesn't make the machine feel slow when some CPU intensive task takes over.
On Mac OS X I thought I would fire up the Activity Monitor and right click to change the priority. Apparently it's not that easy. I fired up google and found out that I need to get the PID from Activity Monitor and then launch a terminal and use the renice command.
E.g. sudo renice +1 -p 3692
A process can have a priority between +20 (lowest priority) and -20 (highest priority). 0 is the default priority. The above will make the the process with PID 3692 with a slightly lower priority.