basically, a NASA/Univerisy of Berkeley project which uses distributed computing techniques to make (arguably) largest supercomputer on earth, composed of small, individual PCs... right now there are 2 million registered users... now, vast majority of them are dormant or put only very few cpu-hours into it, but some of them make up for it by running SETI@HOME on their university or lab Unix-boxes etc when they're not used...
its a scientific program which takes data from Aceribo radio-telescope and searches for meaningful signals... so no, it doesn't do any good to your computer, but if you wanna feel a part of scientific community in some small way (as I do, its a good way to start