Fluxx, from Looney Labs.
You start with three cards, and the rule “Draw one, play one.” Your goal is to find a goal card that matches what you have, such as “Milk and Cookies” or “Appliances.” Along the way, you play cards that change the rules: “Draw two, play three”, “First play random,” and so on.
Some people find it all a little too random; others love it. I think it’s great fun. It’s easy to let someone join you in the middle of a game. It feels like a new game each time, and it’s interesting to ponder which rules bend and which do not. (Reviewed Dec., ’04)