Stamp

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

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 5 are gained by completing every course with any character (1 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. 10 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 prequel, there are 100 Stamps, but none of them are unlocked from the start. 95 of them are earned by obtaining Bits, while the other 5 are unlocked by completing certain challenges.

Mario Kart 8
Stamps also appeared in Mario Kart 8 and in this game, there are 90 Stamps, but 28 of them are unlocked from the start. 30 of them are earned by playing a Grand Prix race (1 per character), and the other 32 are unlocked for beating a Staff Ghost in Time Trials (1 per course).

Captain Toad: Treasure Tracker
Stamps are set to appear in Captain Toad: Treasure Tracker, working as in Super Mario 3D World.

Mario vs. Donkey Kong (Wii U)
Stamps are set to appear in Mario vs. Donkey Kong (Wii U), but how they work is unknown.