Folding@home is a distributed computing system that utilizes your home computer (and now a Playstation3) to help researchers study and cure diseases such as Alzheimer's, Parkinson's, cystic fibrosis, and many cancers. What it does, is install a small program that harnesses some of your computer's spare processing and simulates protein folding. They need all the help that they can get, so volunteering to run in the background of your computer is definitely a good deed.
An added bonus is the fabulous graphics it displays while it is running and that you can use as your screen saver. A bit of art for your desktop (or television), as well as helping to save the world, one protein at a time.