Modern computers are many times more powerful than they need to be for most of the things we use them for. Simply writing papers, surfing the Web, watching videos, playing games, etc. . . such tasks don't take full advantage of these machines' potential and when they're not in use, well . . . they're not in use. They could be doing so much more.
Distributed computing is, as I understand it as a layman, the process by which a massive computing endeavor is divided amongst several different computers networked together, either locally or across the expanse of the Internet. There are a number of worthy projects out there which are seeking volunteers willing to donate computing power and network bandwidth to their causes. Such projects include Stanford's Folding@home which studies protein folding as it relates to disease, IBM's World Community Grid, studying several diseases including cancer, AIDS, muscular dystrophy, to name a few, or even Berkeley's SETI@home, which contributes to the ongoing search for extraterrestrial radio signals of a non-natural origin. All you need to do is go to the project's Web page, download and install the software, and your computer handles the rest.
Many of these projects have been around for several years and have demonstrable results. If you're interested in donating computing resources to any of these projects, check the list at http://boinc.berkeley.edu/projects.php. I've resolved to donate my Core i7's spare time to both SETI and Folding@home (which, by the way, my Playstation 3 also contributes to via the PS3's built-in 'Life with Playstation' software).
So if you're like me and own an over-powered computing rig that you spend more time away from than you'd like, or if you have an older computer that you don't use very much anymore, consider putting it to better use by helping out one or more of these projects.