Via Userfriendly: join the World Community Grid and help save lives - effortlessly. Do I sound like a late-night TV commercial? Now with no added sugar! :-) All jokes aside: WCG is a cool community project which dedicates idle CPU time to things like cancer research. While I don't recommend that you leave on computers just for this (because IMHO it would be a wast of energy), running it during normal use is quite ok (I found the system impact to be minimal). Below you'll find a video which goes into some more details (although - I feel - it is slightly exaggerated in the infomercial direction I mentioned earlier):
A word of warning: for some dubious reasons the install kit requires a reboot to function (???), so you should install it at the beginning/end of your day (or when you plan to reboot your computer). Aside from that it was a really easy install.