SETI@home is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence (SETI). You can participate by running a free program that downloads and analyzes radio telescope data. The program dials up a computer at UC Berkeley and collects a single packet of data at a time for analysis. The data comes from radio signals collected at the Arecibo Radio Observatory. The program runs while your computer is idle (usually whenever your screensaver kicks in). The project has over 4 million participants all dedicating their unused/idle computer time to the analysis of radio signals from outer space that might indicate a source of extraterrestrial intelligence.
I have been a member of SETI@home since September of 1999 and although I barely participated in the first year and a half, I have racked up over 50,000 hours of cumputing time to the project. As of today, I have processed almost 4200 packets of data at an average of 12.5 hrs per packet. I have completed more work units than 98.887% of the others in the project but that still leaves me with 15 others who have a rank of 54,518th place.
I encourage you to participate also at: http://setiathome.ssl.berkeley.edu