There's this great research project going on at http://climateprediction.net where they're calling on the world's idle computers to process chunks of project data pertaining to global warming simulations. If you're running your computer for extended periods of time, this seems like be a great way to serve our planet. They want people with computers who can complete the projects assigned to them, not just people who want a new screen saver or who only run their computers for a few hours a week.
The program can work as a screen saver or not, but basically you have the control to allocate as much CPU time and disc space you want.
You can findout everything you need to know at the above website.
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