Discover Magazine has a great article that highlights some of the things you can do with your computer’s idle time. Most people know about Seti@Home, and Folding@Home, but there are many other great programs you can run to help society.

FightAIDS@Home
What it is: Run by the Olson Laboratory at the Scripps Research Institute, this is the first biomedical and the first humanitarian distributed-computing project.
How it works: Uses computational methods to help identify which drug molecules could best fight HIV.
Our take: It’s a good cause in need of volunteers.

Climateprediction.net
What it is: Roughly $2 billion is spent each year on modeling the impact of global warming. This project attempts to vet the varying predictions.
How it works: The program sifts through thousands of climate models provided by researchers, tossing out the nonsense models as it crunches.
Our take: More than 45 teraflops of data is no joke. Finally, software worthy of your computer’s footprint.

Definitely worth checking out, and I would love to hear more application systems that are using the combined processor power of the world for interesting and good causes.