Many diseases and disorders are caused by one of the basic mechanics of the human body--protein folding. Scientists create and run models of these processes, but it's very computationally intensive. So intensive, that even super-computers don't have enough power to process all the simulations.
The University of Standford runs a program called Folding@Home that combines peoples computers at home into one large super-computer. All you do is install the application and when you aren't using the computer the application starts up and processes a few chunks of information. When you start using your computer again the application backs off. Very slick.
The main downside is that your computer will always be running at full speed. Due to the nature of computers, this doesn't cause any extra stress. But it does generate more heat. This can cause your computer fans to spin faster and be louder. You're also using more electricity...about $14/month worth. And if you live in a warmer climate then you'll notice that a computer running at full speed all day long adds more heat to the room.