I've gone on before about the fact that most computers spend 99% of their processing time waiting for users to do something. There are many projects that allow this wasted capacity to be applied to solving various problems, from detecting aliens to finding cures for cancer. One of the oldest projects is over at distributed.net. They started off trying to win competitions for breaking encrypted messages, but diversified into some hard mathematical problems. This week they completed the OGR-24 project. See their page for the details. I contributed many hours to their projects in the past, but have moved on to other things.