Anyone with access to the Internet can download a program from the SETI site that basically operates in the background, much like a screen saver: when you're not actively using your computer, SETI is. Once every couple of days the program will ask to download a "packet" of data, about 380K, from the SETI server at U.C. Berkeley (just down the street from my apartment, as it happens) then processes that data, typically for a period of about two days. When finished, it asks to upload the processed data to the SETI server and download a new "packet", basically a parallel processing machine with 600,000 CPU's, 600,001 now that I've added mine.

I have no idea if they'll find any Little Green Men, it's a three year project analyzing data being collected by the huge Arecebo radio telescope in Puerto Rico, but it's a great idea: Parceling out the computation to participants all over the Internet, a grass roots project with a few volunteers, not much money and the largest supercomputer in the world.