SETI@home is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data.
It benefits everyone because it assists in the search for extraterrestial life. You just download it and run it and it uses all spare cycles on your computer. Doesn't slow it down either as it just uses time nothing else is using.
I've been running SETI for over a year. Does it work? Well, the software works great and has caused me no issues. Have I found any alien life forms yet - no, but I'm still looking!
You can use wasted CPU cycles to do some useful work, and there are other causes doing the same if you don't like this one. My only "donation" is leaving my PC on all the time so it can process when I'm not around. So I pay the electric bill for 100 watts or so 7 x 24, but I've logged over 3000 hours of CPU time in SETI analysis.
I think it's worth doing this or something else worthwhile with all your wasted computer time.