To me, part of being a geek is being altruistic, whether that means recycling your gadgets or giving back by playing a game. Nothing I have done online makes me feel better than Folding@home.
Folding@home is a download that you allow to run on your computer (that's it — you do pretty much nothing after that) and it uses your unused CPU power (or you can run it as a screensaver) to effectively make a supercomputer to study the folding of proteins. I may not completely understand how protein folding works (check out Wikipedia's very helpful description though), but the professors who do can use it to find cures for Alzheimer's and Parkinson's diseases, and certain kinds of cancers.
This noble cause was started in 2000 by Stanford University professors and was recently named the world's largest distributed computing project by Guinness World Records. And they're into trying to make it fun — they recently introduced a version for Playstation 3!
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