Cap Block

A Cap Block is a type of block that first appears in Super Mario 64. In the original game, Cap Blocks come in three colors, each one containing a different power-up.

Super Mario 64 / Super Mario 64 DS
In Super Mario 64, all Cap Blocks are intangible at the start of the game, reminiscent of the four ! Blocks from Super Mario World. When one of the three Cap Switches is activated, all Cap Blocks of that color solidify and drop a cap power-up upon being broken open. In Super Mario 64 DS, the three colored ! Blocks are reduced to one red ? Block, which is simply referred to as such. Red blocks contain either Power Flowers, which give different abilities depending on which character obtains them, or Wings in certain levels for Mario. If anyone other than Yoshi hits a red block without his cap, however, a Bob-omb will come out instead.
 * Red blocks (or Red Blocks) contain the Wing Cap. The red Wing Cap Switch is on the Tower of the Wing Cap, which is accessed by standing on the sun panel in the Mushroom Castle's lobby and looking at the ceiling after Mario obtains at least 10 Power Stars.
 * Green blocks (or Green Blocks) contain the Metal Cap. The green Metal Cap Switch is in the Cavern of the Metal Cap, a secret area inside the Hazy Maze Cave accessed through a portal near Dorrie's pond.
 * Blue blocks (or Blue Blocks) contain the Vanish Cap. The blue Vanish Cap Switch is in Vanish Cap Under the Moat, which becomes accessible after Mario drains the moat in front of the Mushroom Castle.

A different set of Cap Blocks exists unused in Super Mario 64 DS, and they provide the player with Mario's cap, Luigi's cap, or Wario's cap.

Super Smash Bros. series
The green block appears as an item under the name Metal Box in every installment of the Super Smash Bros. series starting with Super Smash Bros. Melee. Much like in Super Mario 64, this item causes characters to become coated in metal for a short period of time. Unlike in Super Mario 64, they have to simply grab or directly hit the box to become metal, as opposed to jumping under it and causing the Metal Cap to emerge from it. They become heavier and fall faster, making it harder to directly KO them by blasting them away, but making it harder for them get back on the stage. Getting hit causes the duration of the power-up to shorten. The Final Smash forms Giga Bowser, Wario-Man, Mega Lucario, and Giga Mac are completely unaffected by Metal Boxes. Also, when a character becomes metal, their voice is removed.

Every game in the Super Smash Bros. series that features trophies has one of the Metal Boxes. In Super Smash Bros. for Wii U, the trophy erroneously states the Metal Box appears in Super Mario 64 DS, which replaced the colored caps with multiple characters, their Power Flowers all being found in red blocks.

Minecraft
In the Super Mario Mash-up in Minecraft, the red, green, and blue Cap Blocks appear as paintings, along with a yellow block from Super Mario 64.

Super Smash Bros. for Nintendo 3DS / Wii U
In Super Smash Bros. for Wii U, the Metal Box trophy lists Super Mario 64 DS as its other game appearance, even though it (as well as the original game's red and blue ! Blocks) were replaced by ? Blocks in the remake. Blue indicates it is exclusive to the Wii U version.

Trivia

 * In some pre-release versions of New Super Mario Bros., a blue block from Super Mario 64 can be seen. Its 3D geometry is more apparent than any other block in the game. Moreover, the yellow block from the same game can also be seen, this one being automatically switched to a green block if hit.