Parasitic Computing

Siphoning the computational power of the Internet, U.S. scientists have figured out a way to induce unwitting Web servers across the world to perform mathematical calculations. Researchers at the University of Notre Dame in Indiana solved a complex math problem with the unauthorized help of computers in North America, Europe and Asia. Using a remote server, the team divided the problem into packages, each associated with a potential answer. The bits were then hidden inside components of the standard transmission control protocol of the Internet, and sent on their merry way.
http://fyi.cnn.com/2001/TECH/internet/08/29/stealth.computing/index.html