Folding@home

As gamers most of us have got reasonable CPUs and a lot of us leave our machines on idle cause we're lazy or because we're downloading something completely legal over a p2p network.
Well, folding@Home is a way to do something worthwhile with all that spare power sitting around doing nothing. Basically it's a distributed computing network that's used for simulating protein folding, this kind of research has all kinds of potential medical benefits.
Costs you nothing, and really isn't much of a resource hog when you run it in the background.

http://folding.stanford.edu/

You can also get it for your PS3. Which might be pretty cool. One day we might tell our grandkids that we cured cancer with our games consoles.