I made a chess board. It lets you play chess. Wow!
The board allows you to play chess against a friend. It enforces the rules of the game, so nobody can just cheat their way to victory. It features an easy to read display, simple control panels, and a fun song for the winner. All this with only pure redstone, no commands or functions.
If you're interested in how the board works, I made a video that shows some gameplay and how to use the controls, and includes a Completely Legitimate Tutorial™ for how to build one yourself: https://youtu.be/ONm3a307pwc