I think Seti@home has to be one of the best uses for computer idle time. Even when your working on a PC over 90% of the CPU time is idle time. Why not put it to a good use?
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.