Mario Party 4
The Mega Mushroom is a special powerup that first appeared in Mario Party 4 for the Nintendo Gamecube. The item grew a player to a huge size and allowed them to roll two dice at once. The player also was able to step over other players and rob them of 10 coins. The Super version works the same as the normal except that players get to roll the dice 3 times.
New Super Mario Bros.
In New Super Mario Bros. for the Nintendo DS, the Mega Mushroom first appears in level 1-1. It temporarily turns Mario into an extremely large version of himself, about half as tall as the entire DS screen. In a way, the Mega Mushroom replaces the invincibility star as it makes Mario invulnerable to enemies, though he can still fall down a large enough hole. In addition, he also gains the power to destroy blocks and pipes by running through them, essentially making Mario an unstoppable juggernaut.
Mario Kart Wii
The Mega Mushroom also appears as a powerup in Mario Kart Wii. In that game, it turns the player huge and allows them to crush other drivers, but also reduces their speed.