1000 Blank White Cards, a game that’s making the rounds of weblogs lately. Basically, you use blank index cards and draw the rules of the game as you go along, awarding points for certain things.
Here’sanother explanation of the games rules (or lack thereof).
Also, a slide-deck from the creator of the game on Philosophy of Creativity: Finite and Infinite Games.
Remember Calvinball, from Calvin and Hobbes, where the game could never be played the same way twice? That’s the idea here… creative, fun anarchy.
(2014 update: I never managed to get a game together. I still would like to. Also: the Wikipedia page on the game.)