I’ve been a member of World Community Grid for a while. From the site:
World Community Grid’s mission is to create the world’s largest public computing grid to tackle projects that benefit humanity.
Basically, this means that, if you have the client installed and your computer fits the requirements, whenever you computer is idle (i.e. doing nothing) it’s processing power will be used for several projects. At the moment you can choose to work on projects like Help Defeat Cancer, Fight AIDS@Home, Human Proteome Folding and more.
Now, I know many Xubuntu users do not have state-of-the-art hardware, but we too can make a contribution. That’s why I’ve set up a Xubuntu Users team, so we can track how much we contribute.