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 will drop a cap power-up upon being broken open.
 * 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 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 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.

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.

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 in every installment of the Super Smash Bros. series starting with Super Smash Bros. Melee, under the name Metal Box. 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 will cause 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.

From Super Smash Bros. Melee onward until Super Smash Bros. Ultimate, each game of the Super Smash Bros. series has a trophy of the Metal Box. 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, and their Power Flowers are all 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
Blue indicates exclusive to the Wii U version.

Trivia

 * All three Cap Blocks are treated as proper nouns in the Super Mario 64 Player's Guide.