I am way behind on this, but it's been a busy month. :-) SETI is the Search For Extraterrestrial Intelligence. It's a project by a phenomenally dedicated and brilliant group of scientists and engineers to search the sky for radio signals from other civilizations. I won't go into the basis for this here (you can read their FAQ for all that), but I think it's a worthwhile endeavor.
One of their biggest problems is the sheer volume of data they generate. They probe the sky looking at a huge number of radio channels for a signal -- imagine your car radio able to tune in to not 20 or 30 stations, but millions. Billions. That's what they face.
Some very smart folks at Berkeley realized that home computers could be utilized to process some of that data, so they created SETI@Home. When you are not using your PC or Mac, the processor can be utilized to crunch through the SETI data, looking for that needle in a million haystacks: an intelligent signal (good luck finding one on your AM dial!).
This project has been going on for some time (search the BAUT forum for a discussions of it, including out own BA and UT team)
Right now, SETI is getting a surge of data because their main dish, the Arecibo radio telescope in Puerto Rico, got an upgrade. SETI@Home needs more computers!
So please sign up. It's free, doesn't hurt your computer in any way, and if your computer is the one that finds The Signal, then I imagine they'll be happy to let you share the glory, too.
Update: Just to be clear, SETI@home is not a product of the SETI Institute; it's an independent program out of UC Berkeley.