Seti@home : Search for extraterrestrial intelligence, was the first and most popular scientific calculation program that makes use of those millions of pc’s doing nothing.
Now there is fold.it, a sort of game where you have to fold proteins. This is a very complex matter and David Baker invented this game to put not only the pc’s to work, but also the brains of all those humans behind the pc’s.
So it is not only cloud computing, but also cloud thinking !
Besides that there is also Games with a purpose : games that serves some purpose for the people setting up the game by harnessing human abilities in an entertaining setting
Google image labeler uses people( who have nothing better to do) to tag images.
In Galaxy zoo you can participate to the search of new galaxys in space.