I’ve been involved with the World Community Grid for several years and it’s become something of an addiction. I know that there are scores of such projects out there and I’m sure a lot of people here must be involved in one or more of them.
World Community Grid also.
World Community Grid. I figure IBM does a decent job of filtering the projects for software glitches. Here are their current projects. I’m currently supporting solar energy research, fighting prevalent third world diseases and studying water filtration methods. Other work focuses on childhood cancer and protein folding.
Years ago, I had SETI installed on my machine.
The software has been jiggered to be laptop friendly – you can arrange to run the program only when the computer is plugged in, so you don’t drain your battery.
I understand that the combined distributed processing power of BOINC exceeds that of the fastest supercomputers. Though it could still use your help.
I’ve written some very basic cloud applications using Hadoop, and I expect to do more advanced things in the future.
My computer at work runs the University of Leipzig’s FindLinks client. It’s a distributed web crawler, the results of which are used for research on and using large web corpora (which is part of my job), as well as for a website-recommending application called NextLinks (which I don’t use and have actually never tried).
So 85% of the posters on this board are involved with a distributed computing project according to this poll. That’s amazing! 
Incidentally, there’s a SDMB team at World Community Grid. It’s called Teeming Millions, and it was formed in 2004. I see that it has 11 members. World Community Grid - Team