Okay, to be completely honest, I don't know why anyone would want this, but I found a simple way to (nearly) max out your machine's CPU usage. Perhaps it could be used to artificially load your system while testing to see how other apps perform under duress. Use some sort of CPU usage tool (like CPU Monitor) to watch the effect. Ctrl-C or close the Terminal window to stop.
To max out userspace CPU time
yes > /dev/nullTo max out kernel CPU time
cat /dev/zero > /dev/nullI don't own a dual-processor system, but I'm curious what the effect is.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020820012524273