Bartok is a game similar to Crazy Eights or UNO, except that the winner of each round is permitted to announce a new rule which must be followed in all future rounds.

The game is played with one or more decks of cards, and can even be played with mixes of partly complete packs. Differences from standard Crazy Eights are:

If you only have one card left, you have to say 'Bartok'. If you don't, you get your hand made back up to maximum.
Most other rules violations require you to take one or more cards.
Players aren't allowed to ask questions during a game. Ask a question, take a card. This is most entertaining when someone comes along and asks 'What are you playing?'. They'll usually get a card, and then several more when they don't say Bartok.
This is the doozy: A player that successfully gets rid of his last card is deemed the winner for that round, and gets to make a new rule, which overrides any later rules. These can be as simple or as brain-hurty as you like. Usually the game gets harder as it progresses, as rules feed off each other and make lethal feedback loops.

Some example rules, to give you an idea of the flavor of the game:

When a change of suit changes color, play direction reverses. (easy)
When a two is played, play skips a player. (easy)
Threes don't exist. (somewhat metaphysical)
When a black ace is played, each player must slap a card face down in the middle of the table. The last player to do so takes all of the other cards. (moderate, more of a wakey wakey rule)
When a black queen is played, all of the cards increment their values by one, with kings wrapping to become aces. Red queens decrement the values by one. (somewhat mind numbing)
Prime numbered cards swap to become the suit of the same colour - ie. clubs <-> spades, diamonds <-> hearts. (brain hurty, especially when combined with the rule above)

Any rule disputes are usually settled by voting. You play until you're bored, or until someone creates a rule that would explicitly end the entire game (with or without ultimate winners), and the rule is triggered. The game can also end if further play becomes impossible, as a result of how the created rules combine.

SURGEON-GENERAL'S WARNING: Do not play this game with mathematicians :)