The Propeller Mushroom is a power-up that lets you fly for a limited time. Shake the Wii remote and Mario will fly. Also in a 2 or more player game, if you have a Propeller Mushroom and another player doesn't, you can pick them up then shake the Wii remote and you and the other player will both fly. The propeller mushroom also gives Mario some extra health. It has the same affect as the Fire Flower or the Ice Flower. It gives the third stage of health. Therefore, Mario can survive three enemy hits. This power-up is very valuable for getting to secret locations.
200px-Propeller Mushroom

A propeller mushroom