Fluxx is a card game with ever changing rules. The cards themselves determine the rules of the game (and the winning conditions).
Fluxx is by Looney Labs and is reasonably priced at $16. If you like the core game, there are tons of expansions from Wizard of Oz Fluxx to Zombie Fluxx, to Spanish Fluxx, to Family Fluxx. The expansions can be played separately or combined with the core rules.