There are sites around the world that use a massive grid of home computers to help process gargantuan data sets in search of answers to important questions. Two that I know of are SETI@Home and the World Community Grid.
SETI will farm out data sets to your PC to be analyzed for signs of extra-terrestrial life. It uses a client that can run as a screen saver and it looks very cool. You can form groups and communities and it becomes a fun contest to see who can contribute the most processing power :-)
I gave them a lot of days of processing back about 10 years ago, but changed jobs and lost my connection to that community.
The World Community Grid, however, searches for answers more local to home. For example, I processed 4 days worth of Dengue Fever data sets in phase 1 of their search. They just sent a confirmation that this phase has ended a few moments ago.
If you tend to leave your machines on all the time, consider hooking up to one of these two projects and giving something back to the planet ...