Distributed networking or grid computing has proven to be an excellent method of using idle processor time in computers around the world in order to crunch large chunks of data. This allows projects that would have to spend millions in buying top of the line computers to distribute the load with volunteer machines.
A new project is now being advertised by the BBC. This will look at climate change, and the potential effects of human action in the vastly complex global climate system. The system will appear in your computer as a screensaver.
Of course, the most famous grid computing project is SETI@home, the search for extraterrestrial intelligence.