Mega Mushroom

The Mega Mushroom is a rare variety of mushroom found in the Mushroom Kingdom. When eaten, they grow their consumer to a massive size. They debuted in Mario Party 4, where they were depicted as normally proportioned mushrooms with green caps, however their current appearance is quite different and unique amongst the Mushrooms of the Mario series. Introduced in New Super Mario Bros., the current Mega Mushrooms have bulbous, yellow and red-spotted caps.

Mario Party 4
Mega Mushrooms first appeared in Mario Party 4, with a light green cap. It could be obtained by landing on a Mushroom Space, purchasing it from an item shop, getting it from an Item Bag, or winning a mini-game on the board. They let the player roll two Dice Blocks at once, making the player grow huge and crush other players as they stomped across the board, stealing 10 coins from them. However, board events will be bypassed (except for events involving Bowser at Bowser's Gnarly Party). Additionally, a Super Mega Mushroom was available, colored entirely translucent green. The Super Mega Mushroom did the same thing but with three Dice Blocks.

Super Mario 64 DS
While it didn't make an appearance in Super Mario 64 DS, the Super Mushroom has the same effect as a Mega Mushroom, as it makes the character giant.

New Super Mario Bros.
They later reappeared in New Super Mario Bros., where they yet again made Mario grow to a humongous size. In this colossal form, Mario could stomp across the level and crush anything in his path - enemies, blocks, Warp Pipes, and even the flagpole at the end of the level. These mushrooms were rarely found, and almost always came from Toad Houses or Red Flying Blocks. Besides the Toad Houses, they only appear in two levels (without exploring and finding secret areas within the level), 1-1 and 7-5. The player is not 100% invicible in this form, however; if the player was to jump and land on a hole covered with bricks, the player would fall through, thus, losing a life, if the hole is big enough. Additionally, if Mario or Luigi were to do a Ground Pound after eating a Mega Mushroom, a group of Goombas or Coins (or both) would fall from the sky.

Mario Kart Wii
In Mario Kart Wii, the Mega Mushroom is an item obtained from the Item Box. When a player uses this item, he/she becomes bigger, able to crush other players for a certain amount of time, reducing their speed. While this item is active, the player won't slow down when going off-road, and will be immune to all items, except for the Star, Bullet Bill, Blooper, and Thunderbolt. Should a player get hit by a Thunderbolt while he/she is big, he/she will shrink back down to normal size rather than become tiny. The Mega Mushroom's theme music from New Super Mario Bros. is played for the duration of the item's effect.

Mario & Luigi: Bowser's Inside Story
In Mario & Luigi: Bowser's Inside Story, Broque sometimes attacks by using a Mega Mushroom to make himself giant. Bowser would've needed to punch Broque's body to avoid the attack.

Trivia

 * The Mega Mushroom uses the same color scheme as the original Super Mushroom in Super Mario Bros. and Super Mario Bros.: The Lost Levels.
 * In New Super Mario Bros., When Mario is under the effect of the Mega Mushroom the running animation is similar to the running animation of Super Mario Bros..
 * In Mario & Luigi: Partners in Time, when Dr. Shroobs eat normal mushrooms, they turn giant.
 * The Fire Yellow Toad in New Super Mario Bros. Wii has a coloration very similar to the Mega Mushroom as his cap turns yellow and his spots become red. However, the Mega Mushroom itself is absent in this game.