Stamp

Stamps are collectibles found in many Wii U games. They are earned by completing certain in-game tasks, and can be used on handwritten Miiverse posts.

As of November 7, 2017, however, the termination of the Miiverse service renders all stamps unusable.

Super Mario 3D World
Stamps first appeared in Super Mario 3D World. There are 85 stamps in this game. 80 of them are scattered in levels or Sprixie Houses, while the other five are gained by completing every course with any character (one gained per character).

NES Remix
Stamps make their second appearance in NES Remix, where there are 100 Stamps in it. They are gained by obtaining enough Bits. Ten are unlocked from the start, while 90 must be unlocked.

NES Remix 2
In the sequel NES Remix 2 they appear once more, and like in the previous game, there are 100 Stamps, but none of them are unlocked from the start. 95 of them are earned by obtaining Bits, while the other five are unlocked by completing certain challenges.

Mario Kart 8
Stamps also appeared in Mario Kart 8 and in this game, there are 100 Stamps, but 28 of them are unlocked from the start. 30 of them are earned by playing a Grand Prix race (one per character), and the other 32 are unlocked for beating a Staff Ghost in Time Trials (one per course). In addition, ten more stamps were added if they purchase both DLC packs. Stamps do not make a return in Mario Kart 8 Deluxe.

Mario vs. Donkey Kong: Tipping Stars
Stamps appear in Mario vs. Donkey Kong: Tipping Stars. One way of getting those is by buying items in the Workshop Store. Over 70 stamps can be collected.

Yoshi's Woolly World
Stamps appear in Yoshi's Woolly World. In this game, every level hides 20 Stamp Patches inside beads, the game's collectable currency. Collecting a certain amount of these Stamp Patches will earn the player five new stamps. When a Stamp Patch is collected, the player will be able to see a stamp icon on the beads they collected them from, whenever they replay the level. There are 70 stamps in total to collect.

Super Mario Maker
Stamps appear in Super Mario Maker. In this game, they are available by default, and no in-game tasks are required to unlock them.

Mario & Sonic at the Rio 2016 Olympic Games
Stamps appear in Mario & Sonic at the Rio 2016 Olympic Games. In this game, there are 100 stamps for players to collect.

Super Mario 3D World

 * Instruction manual description: "You can use the stamps you collect in your Miiverse posts."