This is not about alien invaders or anything like it, but SETI has a website where you can download a program which analyses the data from the Aricebo radio telescope. It's a great setup called SETI@HOME which lets you use your computer's down time to analyse these signals (almost entirely radio noise at this point). The idea is to send packets of data to about 3 million computers therby forming in effect a supercomputer. As my machines are always on, I have done about 600 hours of computing for them.
Hey, it doesn't cost anything and who knows...
The link is:
SETI @ HOME