I know a lot of you run SETI@home on your machines but did you know you can use the same technique to support a wide variety of charitable causes, too? Distributed computing is currently used by groups like the SETI Institute and the National Foundation for Cancer Research to do massive number crunching that would otherwise be prohibitively expensive.
Participants install a small program that runs in the background. The program downloads data from the Internet, crunches the numbers when your machine is idle, then uploads the results when it's done.
Take a look at these pages and see if there's a project you want to support. It's a great and painless way to make a big difference.