BOINC and distributed computing
I've been runing distributed computing projects on my computers for several years now. I started out with SETI@Home (which was the first of its kind and is still extremely popular), then I switched to D2OL (drug design and optimization) and now I decided to switch to one of the projects that run using the special BOINC client. There are several projects to choose from and I decided to join two of them for now: ClimatePrediction.net and LHC@Home (helping to build the Large Hadron Collider). I will also join Einstein@Home (helping to detect gravitational waves) when it becomes available. If you have some excess computational power, I encourage you to donate it to the science.