I think Seti pioneered the idea of distributed computing and really put its foot in the door, but as far as my personal belief that its really a useful tool, I don't think so. I feel products like Folding@Home, ZetaGrid, and that Cancer one are much better applications of distributed resources.
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.