Over the weekend, I was testing how some of our apps work when the CPUs are busy. One way to load the CPUs is to rip a Blu-ray disc, but I was looking for a more controllable CPU load.
More specifically, use this command in Terminal:
yes > /dev/null &
If there's an award for strangest Unix utility, yes might just be the winner. All it does is output y (or whatever you list after the y; the man page suggests an expletive) until you kill the task.
The above command sends the output (via the > redirect) to the null device, which discards it. The ampersand sends the job to the background, so you get your Terminal prompt back.
You can run this command multiple times, each loading the CPU even more heavily (the screenshot shows three yes tasks running). Keep an eye on Activity Monitor to see just how much CPU it takes—as shown above, it does a great job at loading the CPU.
You can kill the tasks by issuing the command killall yes in Terminal, or by quitting Terminal—you'll be told that quitting will terminate the tasks.