I was doing the distributed.net challenges for the longest time, but gave up after they proved most of the cryptography challenges. I was going to do SETI@Home but gave up that idea when I learned there were groups cheating their scores, making it look like they cracked more space than they really did, and besides ET will contact us soon enough.
I've been running the Grid.org software looking for a cure for cancer for a few years, I just checked and see that I have donated 5 years of computing time with my computers.
