From Scientific American Magazine:
What is Citizen Science?
Research often involves teams of scientists collaborating across continents. Now, using the power of the Internet, non-specialists are participating, too. Citizen Science falls into many categories. A pioneering project wasSETI@Home, which has harnessed the idle computing time of millions of participants in the search for extraterrestrial life. Citizen scientists also act as volunteer classifiers of heavenly objects, such as in Galaxy Zoo. They make observations of the natural world, as in The Great Sunflower Project. And they even solve puzzles to design proteins, such as FoldIt. We’ll add projects regularly—and please tell us about others you like as well.
List of Citizen Science Projects here.