ClimatePrediction.net
Not exactly a green topic, but close…
…is one of those donate your unused cycles for massive computation projects. Of course, if you leave your PC on all the time to contribute to this…
I wonder if there is a way to combine tasks like virus-scanning (for Windows boxes) or other long running tasks with these net-parallel computation tasks that lets you sleep or hibernate your computer after the main task (say virus scanning or disk defragmentation is completed).