Mega Mario

Mega Mario is a form that Mario and some others can become by eating a Mega Mushroom in many games. This form allows the player to greatly increase in size, allowing them to defeat every enemy in his path and break almost anything (including Flagpoles).

Mario Party 3
In the mini-game Toadstool Titan, characters could obtain the mega form from taking a Mushroom hidden in one of the Brick Blocks. Once the character had obtained the mushroom he or she was capable of knocking out other characters in a short period of time. Mega characters could not jump and control their moves, however.

Mario Party 4
In this game, the Mega Mushroom cause one to enlarge in size and hit two Dice Blocks. While Mega, you can take 10 coins from any opponent you squish.

Super Smash Bros. series
In Super Smash Bros. Melee and Brawl, any character can become Mega by acquiring a Mushroom. When a player is Mega, his or her attack power increases by a fair bit. In Brawl, the final event match pits you against Mega Mario, Snake, and Sonic. Both games also offer an option to play as Mega for an entire match. Also Jigglypuff's final smash turns is transforming large as if it had used a mega mushroom.

Super Mario 64 DS
In Super Mario 64 DS, the Super Mushroom has the same effects as the Mega Mushroom, and makes the player into a larger size. The player can then obliterate almost anything in their path. They are always found in Bricks scattered throughout the level, and were first introduced in Bob-Omb Battlefield. The player however, could not do any special types of jumps, such as the Triple Jump. If the player breaks more than 8 things, he'll get a 1-up for every extra object he destroys.

New Super Mario Bros.
The Mega Mario form returns in New Super Mario Bros., and is very useful sometimes. The more things Mega Mario breaks, the higher the bar above his head will rise. If done right, Mega Mario can gain up to five 1-Up Mushrooms. Mega Mario also has the ability to squash any of New Super Mario Bros.' bosses in one hit, including Bowser in any of his forms. When Mega Mario falls into the purple toxic water or lava, he immediately shrinks back to Super Mario and dies. Also, if he become Mega Mario in tight spaces, he'll return to his original form and the Mega Mushroom will return to the touch screen.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Mario or his partners can become huge by using a Power Punch. This increases their attack power briefly for a few turns.

Super Paper Mario
In Super Paper Mario, when Mario (or Luigi, Princess Peach, or Bowser) grabs a Mega Star, he will turn 8-bit, much like his sprite from Super Mario Bros and grow to giant proportions as well. As Mega Mario, Mario will be able to break blocks, enemies and anything in his path, much like the Mega Form from New Super Mario Bros.

Mario Super Sluggers
In Mario Super Sluggers, any character can become temporarily Mega by finding a Mushroom in the mini-game Graffiti Runner. When a player is giant, he or she can create small earthquakes that can paralyze all ground opponents for a brief period.

Mario Kart Wii
In this game, the Mega Mushroom appears as an item. When utilized, the character becomes gigantic, allowing him or her to squish opponents and get a speed boost. However, the player is not immune to Thunderbolts or Bullet Bills.

Trivia

 * According to The InterNed, the music in Super Paper Mario that plays while Mario or another character (including a Koopa) is in Mega form is titled "An Unrivaled Battle."
 * A similar form to Mega Mario, though unnamed, appeared in the Club Nintendo comic "Mario in Mariozilla". Kamek accidentally turned him into this state.
 * In Super Mario 64 DS, the only time when Yoshi can punch is while he is in his Mega state.