Since the guys at SETI came with the idea of using the CPU time of internet users when they are not using their computers, several other projects draw inspiration from this idea to obtain some sort of highly distributed high performance computing when the funding is not there.
One of these projects is called the world community grid which involves many research centres and universities and tries to tackle several problems that should be of general concern. One of the projects is about Cancer. One of the ways to go about cancer research is by using tissue microarrays in which samples of tumour cells are treated differently and the results of the different treatments can be obtained and compared in a comparatively efficient way. I am writing this from Columbus airport but when I get the chance of getting back to Dresden I should install this client on my Linux workstation. They do have versions for Linux, Mac and Windows.
Of course one thought is that if I know that I will not use the computer in a while the right thing to do (assuming one cares about the world) is to switch the computer off but I guess that those times in which the screen saver kicks in I would be happier thinking that my computer is doing something interesting instead of just displaying pointless and CPU intensive openGL pictures.