I am a big fan of what is sometimes called grid computing. In essence, a master computer doles out work to other computers on the network, which would do some portion of the processing, return results, and fetch a new work unit. Probably the best known effort on these lines is the SETI at Home project.
The effort that I am really into though is Stanford's Folding at Home. It is a grid or distributed computing project that is investigating proteins and how they assemble themselves or fold. Through this effort, they are hoping to understand and maybe eventually cure dieases like cancer, Alzheimer's, and Parkinson's. The client software can be installed either as a screensaver or as a service (I recommend service). It runs in the background on a low priority and the system software takes care of making sure that the user has good performance with his real work.
The program works with Windows PCs, Macintosh, or Linux. I am a member of Team 18 MacAddict4Life.
This is a really worthy effort. I encourage everyone with a computer to participate.