ClimatePrediction.Net

If your computer is idling while you're away, you can consider joining ClimatePrediction.Net to donate a portion of your computer's CPU (while you're not using it) to process the vast data accumulated to figure out what effects certain environmental factors will likely have on the climate.

It may be similar to the Japanese supercomputer that imitates Earth, but climateprediction.net is devoted to distributed computing, harnessing a million computers to do what one supercomputer does.