I have noticed from time to time Safari would go nuts and suck up 80-90% of the CPU and send the temps up and crank up the fan. I would then have to go on a witch hunt through the numerous tabs I had open and try and find the ad banner that's going nuts and close it (or just kill Safari). I started poking around and found http://burgersoftware.com/en/safariadblock to do a nice job in reducing Safari's CPU utilization.