I run SETI@home because I firmly believe that there is life out there. The Universe is too big for the accident of life not to have happened elsewhere.
I think BOINC in general is a brilliant idea, about the only way most people can help in the search for intelligent life, combat climate change or investigate the universe and human genome. It is also about the only worthwile thing most people will do with their computers :) Apart from play Eve-Online ;)
SETI@home Classic was a fantastic project in its time. It popularised distributed computing to an extent unimagined, and laid the groundwork for everything happening now. But (to mangle several sayings) the time has come to stop throwing our toys out the cot and leave the cradle.
SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.