My current screensaver is the SETI@Home program, which uses those ununsed CPU cycles -- the ones that would otherwise be drawing pictures of pipes or scrolling marquees or a Star Trek-like scrolling starfield -- to process data collected by radio telescope, in hopes of identifying possible signals from alien civilizations. I am a big fan of the SETI project, and I am thrilled to have this opportunity to help.
This kind of "distributed computing" project is not unique. There are many such projects out there, harnessing computer time donated by regular people to process data. Using a single computer to process these data would require a machine of enormous computing power, and many of these peojects do not have the funding to secure sufficient time on, say, a Cray mainframe or a Big Blue machine to do the work. Thus, they break it up into small chunks and farm it out to people with smaller machines like the Hewlett-Packard with 1.3 GHz Athlon processor sitting on my desk. It's a wonderfully fun way to participate in a cause in which I believe strongly.
SETI@Home may be the most famous distributed computing project, but it is by no means the only one. If looking for LGM's isn't your speed, you may find a more interesting project here.