Companies like Google are well known for their approach to computing power: using lots of little computers to do the job of a supercomputer. It’s cheap, fault-tolerant, and if done right, almost infinitely expandable. Well if that works with computers networked directly to each other, why not over the Internet?
BOINC (or the Berkeley Open Infrastructure for Network Computing) is a software system designed to coordinate CPU sharing over the Internet. There are BOINC client programs that allow you to donate your unused processor power to a variety of purposes, largely non-profit projects and organisations.