2 to 12 players
A standard 52-card deck.
To win all of the cards.
Choose a dealer randomly. The dealer shuffles the cards and deals them as evenly as possible to all of the players. It's fine for some players to have one card more than other players.
Each player places his cards, face down, in a pile in front of him.
The player to the left of the dealer goes first. Play then moves clockwise.
On his turn, each player turns over the top card from his face-down pile. When someone turns over a card that matches a card already face up on another player's pile, players race to be the first to call "Snap!". The player who calls "Snap!" first wins both piles and adds them to the bottom of his face-down pile.
When moving a card from his face-down pile to his face-up pile, each player should do so by turning the card away from himself. This ensures that the player does not see the card before his opponents have a chance to do the same. (The player should also turn the card quickly, so that he's not giving himself a disadvantage.)
If two players call "Snap!" at the same time, the two piles are placed in the center of the table and combined into a face-up Snap Pot with one of the two matching cards on top. Play then continues where it left off.
When someone turns over a card that matches the card on top of the Snap Pot, players race to be the first to call "Snap Pot!". The player who calls "Snap Pot!" first wins the Snap Pot and the matching pile. If there's another tie, the matching pile is added to the Snap Pot.
If you find that your games of Snap tend to have a lot of ties, consider putting something in the middle of the table that players must put their hand on top of instead of calling "Snap!". (An index card with "Snap!" written on it would work just fine.) The first player to put their hand on top of the index card wins the cards.
Making a Mistake
When a player calls "Snap!" or "Snap Pot!" at the wrong time, or puts his hand on top of the index card at the wrong time, he must give his top card to the player who just played. If a player makes a mistake on his own turn, he must give his top card to the player on his right.
Play continues until one player wins all of the cards. That player wins the game.
NOTE: It's possible that some cards will remain in the Snap Pot when all the other cards have been claimed. (e.g., Two of the 10s are claimed by Alexandra and later in the game the other two 10s are involved in a tied call of "Snap!".) If this happens, the winner is the player who wins all the cards not stuck in the Snap Pot.