SETI@home ("SETI at home") is a distributed computing (grid computing) project using Internet-connected computers, hosted by the Space Sciences Laboratory, at the University of California, Berkeley, in the United States
There were two original goals of SETI@home. The first was to prove the viability and practicality of the 'distributed grid computing' concept, and the second was to do useful scientific work by supporting an observational analysis to detect intelligent life outside Earth.
The first of these goals is generally considered to have succeeded completely. The current BOINC environment, a development of the original SETI@home, is providing support for several computationally intensive projects in a wide range of disciplines.
The second of these goals has failed to date: no evidence for ETI signals has been shown via SETI@home. However, ongoing continuation is predicated on the assumption that the observational analysis is not an 'ill-posed' one. The remainder of this article deals specifically with the original SETI@home observations/analysis.
SETI@home searches for possible evidence of radio transmissions from extraterrestrial intelligence using observational data from the Arecibo radio telescope. The data are taken 'piggyback' or 'passively' while the telescope is used for other scientific programs. The data are digitized, stored, and sent to the SETI@home facility. The data are then parsed into small chunks in frequency and time, and analyzed, using software, to search for any signals--that is, variations which cannot be ascribed to noise, and contain information. The crux of SETI@home is to have each chunk of data, from the millions of chunks resulting, analyzed off-site by home computers, and then have the software results reported back. Thus what appears an onerous problem in data analysis is reduced to a reasonable one by aid from a large, Internet-based community.
The software searches for four types of signals that distinguish them from noise:
Spikes in power spectra
Gaussian rises and falls in transmission power, possibly representing the telescope beam's main lobe passing over a radio source
Triplets — three power spikes in a row
Pulsing signals that possibly represent a narrowband digital-style transmission
There are many variations on how an ETI signal may be affected by the interstellar medium, and by relative motion of its origin compared to Earth. The potential 'signal' is thus processed in a number of ways (although not testing all detection methods nor scenarios) to ensure the highest likelihood of distinguishing it from the scintillating noise already present in all directions of outer space. For instance, another planet is very likely to be moving at a speed and acceleration with respect to Earth, and that will shift the frequency, over time, of the potential 'signal'. Checking for this through processing is done, to an extent, in the SETI@home software.
The process is somewhat like tuning a radio to various channels, and looking at the signal strength meter. If the strength of the signal goes up, that gets attention. More technically, it involves a lot of digital signal processing, mostly discrete Fourier transforms at various chirp rates and durations.
While the project has not detected any ETI signals (see extraterrestrial intelligence), it has identified several candidate targets (sky positions), where the spike in intensity is not easily explained as noisespots for further analysis. The most significant candidate signal to date was announced on September 1, 2004, named Radio source SHGb02+14a.
Astronomer Seth Shostak (2004) has stated that he expects to get a conclusive signal and proof of alien contact between 2020 and 2025, based on the Drake equation. This implies that a prolonged effort may benefit SETI@home, despite its (present) nearly ten-year run without success in ETI detection.
While the project hasn't reached the goal of finding extraterrestrial intelligence, it has proved to the scientific community that distributed computing projects using Internet-connected computers can succeed as a viable analysis tool, and even beat the largest supercomputers.. However, it has not been demonstrated that the order of magnitude excess in computers used, many outside the home (the original intent was to use 50,000-100,000 "home" computers)  has benefited the project scientifically.
Eek! 2020-2025! Plus all these other programs that are being sent out to search the soil deep on Mars and far off planets! We could prove the existence of ETI in our lifetime. And by we, I mean myself included. So, download SETI@home yourself, and then YOU can say "we" as well. Hoorah for you!