BOINC

BOINC is a multiplatform application that lets you donate your computer's idle time to use its calculation power in support of different scientific research projects.

Among others, BOINC collaborates with SETI@Home (looks for life outside our planet), Rosetta@Home (research on proteins to cure different diseases), and Climateprediction.net (predicts the effects of global warming).

You can join as many projects as you like. The interface in BOINC lets you follow the development of the experiments with graphs. You can also set it up to run as a screensaver.

BOINC saves statistics about your contributions, which you can compare with those of other people around the world who are also collaborating on these research projects.