"My Brain Hurts" is a free game by Tom Jolly, played with index cards. Players write the cards; the cards can change the rules.
http://www.silcom.com/~tomjolly/mybrain.htm
From the website:
"The basic idea behind the game is that you have 10 points, and are supposed to have more points than anyone else by the end of the game. Each player gets 3 cards, then writes whatever instructions they want on their 3 cards. Then, everyone's cards are tossed together and redealt out, 3 each, to the players. The cards are then played, the players following the instructions that they just wrote in whatever fashion you deem advantageous to yourself, keeping in mind that other players very likely have the 3 cards you wrote up. The game has some very strict rules to follow, but you should be aware that instructions that you write on the cards can also change these rules."