Threes! -- the careful 14-month original. Pay for it. It's the version made by people who hoped you'd put it down sometimes.
2048 by Gabriele Cirulli -- the canonical version. Open source, free, in any browser tab.
2048 Numberwang -- a parody variant where merges resolve by absurdist British game-show logic instead of arithmetic. Worth a few rounds for the joke alone.
Next: how this is built -- the same game on top of event-sourcing, SSE, and view transitions.
