Do you leave your computer on all day? Do you have extra computers just sitting around?
Did you know you can help science understand protein folding with all those extra cycles? Understanding protein folding may help us find cures for Cancer, Alzheimer's, and many other diseases.
Since I built the new server with a Dual Core processor, I dedicated one of the cores to folding. My personal PC has it as well, albeit much slower since I use it.
It doesn't interfere with using your computer (for an example, you're reading this web page delivered by a program running on the same computer as a folding application). And it helps build a super computer that could never exist otherwise.
Plus, when the application becomes sentient and takes over the world, it might look in favor of those who helped it and not enslave us.