Airborne (those little packages you see at the end of store aisles), first claimed to prevent colds, then claimed to boost your immune system. People who travel a lot think it is going to prevent them from catching diseases while on planes, trains, etc.
Last year, 2009, Airborne settled a lawsuit in which it agreed to pay over $23 million in fines for false advertising. The suit said there is no factual evidence to back the company claims, and said it is like a placebo. Caveat Emptor!