Seti @ Home is a great project that has been going on for years - basically, it uses your home computer when you're not using it to crunch data from radio telescopes. Multiply this by thousands, if not hundreds of thousands of otherwise dormant computers, and you have quite a bit of processing power at your disposal.

I like this concept better, rather than fool people with the fact that the absence of color on an LCD or a CRT = power savings, use the resources we have more efficiently.