After listening to one of my favorite podcast/netcast (www.linuxoutlaws.com), I learned about this greate distrubuted computing project called Folding@home. Basically it is a program that you run on your Windows, Linux, Mac, or PS3 that will do protein folding simulations. The real point of this is to help scientists understand disease through these protein folds.
They did a great thing which is let you register a name so that you can see how many work units you have completed. They chart these results so you can see how well you are doing compared to other people. If you got a few friends, you can even create your own team which also gets placed on a chart. Here's the cream of the crop though, you can print out a certifcate that includes how many work units you've done! It can't get better than that, aside from the great good you are doing with your spare CPU cycles.
So for you folks with Dual Core CPU's, you can run this simulation on one of them and you probably won't know the difference. If you are playing games though, you might want to turn this off during that time. I only have a single core P4 and I don't notice any slowdowns at all so your mileage may vary. Give it a try!
More information at the folding@home website.