A Knight's Colour Tour With Friends
The Knight's Tour is a chess problem that has been keeping mathematicians and puzzle enthusiasts occupied for roughly a thousand years. The challenge is to move a knight across a board so that it visits every square exactly once. It is one of those problems that sounds straightforward until you actually try it. This is a colourful variation on that idea, with an added constraint: instead of visiting every square, you must visit every colour-coded region exactly once, in valid knight moves, with no backtracking.
The interesting part is the level builder. You can generate a random puzzle procedurally, with the engine guaranteeing exactly one valid solution, or you can plot your own path manually and paint the board around it. If your design accidentally allows more than one solution, the engine flags the ambiguous squares and shows you where the logic breaks. Getting a clean, unique solution can be an iterative process, which is either satisfying or maddening depending on your temperament.
Once you have a verified board, sharing it requires nothing more than a link. The entire board state is encoded directly into the URL, so there are no accounts, no databases, and nothing to sign up for. Your contact opens the link and the puzzle is waiting for them. There is also a Solve tab that animates the correct path step by step, which is useful if a friend sends you something fiendish and you would rather understand the solution than stare at it indefinitely.
The whole thing is a single HTML file, vanilla JavaScript, and no dependencies. It runs entirely in the browser.