The ToothyGDL is a way of describing games by listing the choices a player can legally make at each step of the game, and how they affect the state of the game. MoonShadow's compiler turns such descriptions into functional games which can be played in a browser using JavaScript. It should, theoretically, be capable of describing most board and card games and similar puzzles and MoonShadow has recently been working on adding real time aspects.
Perhaps most importantly, it is integrated into the ToothyWiki, allowing you to share your games with the other Wikizens.