If you're not too worried about your PS3 overheating, you can consider donating your spare cycles to humanity by running Folding@home. Since the PS3 client has been released, it has topped the list of contributors by TeraFlops, according to this article.
Folding@home is a project that allows researchers to run simulations involving protein folding. The goal is to better understand diseases such as Alzheimer's and cancer. By allowing researchers to run specially written code components on their computers via the Folding@home clients, a large number of people can each give a fairly small amount of CPU time to the project to create a large pool of CPU resources in aggregate. Think: distributed supercomputer.
By the way, there are clients for a variety of platforms, including versions that run as your screensaver. [Download Link]