Mega Mario is a form that Mario takes over in the game New Super Mario Bros. for the Nintendo DS. Mario will turn into Mega Mario once he finds a Mega Mushroom. Once he does get one, he will be giant sized, being able ram and break almost anything. This only lasts for a few seconds, though, so it is best used wisely.
The Mushroom made a return in Mario Kart Wii, growing the driver to a giant and able to run over any other drivers. This form is also immune to other attacks except for the blue mushroom and the lightning, in which case the driver won't shrink to smaller than normal, but back to normal.