Mini Mushroom

In Mario Party 4 there are Mini Mushrooms that make characters smaller. With this they can fit through special pipes, although the player's dice block is limited to rolls between 1 and 5.

New Super Mario Bros.
In New Super Mario Bros., they make Mario or Luigi very small, thus weakening their gravitational pull and allowing them to jump even higher. If the bosses of World 2 and 5 are defeated by Mini Mario, worlds 4 and 7 are unlocked. Mini-Mario can also access certain hidden areas, filled with Tiny Goombas