Fluxx

Fluxx is a card game in which the rules themselves are determined by the cards people play. Each card has a new rule or goal to win or means of winning. As people play cards, the rules and victory conditions change.

Links:

Official Site

http://wunderland.com/LooneyLabs/Fluxx/Default.html

Wikipedia

http://en.wikipedia.org/wiki/Fluxx

BoardGameGeek.com

http://www.boardgamegeek.com/game/258