Volunteer grid computing projects

Most people have heard of SETI@Home, the volunteer distributed grid computing project in which computer owners let software run on their machine when it is idle (especially at night) that helps search through electromagnetic data from space in an effort to find communications from extra-terrestials. But this is only one of many such projects; over a dozen are described in “Volunteer Computer Grids: Beyond SETI@home” by Michael Muchmore, many of them devoted to health applications.
Why do people donate their computer cycles. At first glance, why not? These programs, most of which run BOINC (Berkeley Open iNfrastructure for Networked Computing), are careful to only use CPU cycles not in demand by the computer owner’s software, so the cycles donated are free, right? Well, sort of, but it takes time to download and install the software, there is some risk of infecting one’s machine with a virus, many users may perceive some risk that the CPU demands will infringe on their own use, etc. Most users will believe there is some amount of cost.
With certain projects, volunteers may get some pleasure or entertainment value out of participating: for example, the search for large Mersennes primes is exciting to those who enjoy number theory; searching for alien intelligence probably provides a thrill to many.
I suspect a related motivation is sufficient for most volunteers: the projects generally have a socially valuable goal, so people can feel like they are helping make the world a better place, at a rather small cost to themselves. For example there are projects to screen cancer drugs, search for medications for tuberous sclerosis, and help calibrate the Large Hadron Collider (for physics research). As Muchmore writes, “a couple of the projects—Ubero and Gómez—will pay you a pittance for your processing time. But wouldn’t you feel better curing cancer or AIDS?”
These projects appear to attract a lot of volunteerism. Muchmore reports estimates of participation that range from one to over five million computers at any given moment. According to the BOINC project, volunteers are generating about 400 teraflops/second of processing, far more than the 280 tps that the largest operational supercomputer can provide.