Over on Play This Thing, Manifesto Games founder Greg Costikyan reviews Fluxx, an underground hit card-game that explores the notion of rules — like Nomic or Calvinball from Calvin and Hobbes — and has sold over 350,000 units. It sounds insanely fun:
Here's Fluxx's base rules: Draw one, play one. Okay, it's a bit more complicated than that; you shuffle the cards, deal three to each player, and when it is your turn, you draw a card from the deck and play one. Initially, there is no win condition.
Some cards are "keepers," which you play to the table in front of you and keep. Keepers have names (e.g., "Bread"), but mean nothing in themselves–but if a Goal card has been played, it stipulates a win condition (e.g., "the player who has both Bread and Chocolate wins"). Naturally, only one Goal is in effect at any time, but a player can change the game's Goal by playing a new Goal card.
Similarly, rules cards can be played to change the rules–increasing or decreasing the number of cards drawn and/or played each turn, and establishing a "hand limit" (e.g., a hand limit of 3 means you must discard any cards you possess above three at the end of your turn). Other cards are "instants," meaning you play them and follow the instructions, after which they are discarded–typically, these might allow you to take a keeper from another player or the like.