BOINC, or Berkeley Open Infrastructure for Network Computing, is an open-source software platform for computing using volunteered resources. BOINC is a program that lets you donate your idle computer time to science projects like SETI @home, Climateprediction.net, Rosetta @home, World Community Grid, and many others. After installing BOINC on your computer, you can connect it to as many of these projects as you like.
I've been participating in BOINC projects since 2008, and my computer has even discovered a new Prime Number via the PrimeGrid project. I think it's a wonderful way to put your computer's idle time to use in the advancement of science. To borrow from the title of a BIll Watterson Calvin and Hobbes comic collection, 'Scientific Progress Goes "BOINC"'!