Lemma, by Kadon

Lemma is a curious game, related to Nomic. At the beginning of the game, there is board, arranged into coloured regions, a set of 48 wooden counters in three colours, and no rules. On each turn, a player must propose a new rule, then take an action with a counter which demonstrates the new rule proposed. If the rule is consistent with all other rules proposed so far, and the move is legal, then the turn passes to the next player, otherwise, the player is eliminated from the game. The last person to take a legal turn wins.

That's about it for rules. The game itself is completely different each time you play, so there's not much more I can say in the way of description, either. From past experience, however, games last for about 45 minutes, and can range from painful to hilarious, depending on the creativity and personalities of the players.

A quick example:

1st player: "Counters may only be placed in regions of a different colour",
             Places red piece in green region.
2nd player: "Pieces may be not placed adjacent to existing pieces"
             Places red piece in yellow region.
3rd player: "Only red pieces may be placed on the board"
             Places red piece in black region.

... and so the game goes on. The rules can become restrictive quite quickly, but it's hard to pin down an opponent completely; for example, you may think that the game is going to be over when the last red piece is placed, but no-one has ruled out EXCHANGING red pieces for green pieces, for example...