SETI@home is a scientific experiment that harnesses the power of hundreds of thousands of Internet-connected computers in the search for extraterrestrial intelligence (SETI).
You can participate by running this screensaver, which downloads and analyzes radio-telescope data. There's a small but captivating possibility that your computer will detect the faint murmur of a civilization beyond Earth.
Seti@Home now comes bundled with the BOINC project. The BOINC project 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.