I'm crunching http://setiathome.ssl.berkeley.edu/ right now. Their premises about the shape of alien signals are questionable, but it definitely looks more worthwhile than brute-force cryptoattack a la distributed.net. The Linux code is nice, but it hogs about 10 MBytes of core. (There is a crontab option to run it during idle hours, though). The rest of the world can run it as a screenblanker.