Arduboy is a miniature game system the size of a credit card. Installed with a classic 8-bit game and can be reprogrammed from a library of open source games available online. Arduboy is open source so you can learn to code and create your own games.
Source: The Developer Website.
The Arduboy is a handheld game console with open source software, based on the Arduino hardware platform.
The original version of the Arduboy was 1.6mm thick, with the height and width of a credit card, and was initially designed by Kevin Bates as an electronic business card.
Later consumer versions replaced the first version's touch-sensitive panels by physical buttons, and include a protective plastic case, raising the thickness to 5mm. As well as the open-source Arduboy itself, a single-game version featuring an officially licensed (non-open) version of Tetris is also available. Development was funded through a Kickstarter campaign in 2015.
Source: Wikipedia, "Arduboy", available under the CC-BY-SA License.