There are at least two versions of Slapjack. Both the versions I know involve slapping cards.
Both versions are played with a standard 52-card pack and can be played by any reasonable number of players from 2 upwards. They are probably best for about 4-10 players.
Divide the pack roughly equally between the players. Everyone holds their cards as a face down pile. Going around the table clockwise, each player paces the top card of their pile face up in the centre of the table, making a face-up pile. When someone plays a jack, the first player to slap it - i.e. put their hand palm down on top of the centre pile - wins the centre pile and adds these cards face down to the bottom of their own pile. Play then continues starting with the player to the left of the one who won the cards.
When playing a card you must not peek at it in advance; you must turn it over so that everyone sees it at once. It is best if the card is moved to the centre of the table face down and then turned "away from" the player (so that if anything the others will see it a split second before the player).
If several people slap, the person whose hand is underneath (i.e. in contact with the card) is the winner of the cards. A referee might be needed if you are very competitive about this.
If a player slaps a card that is not a jack, the player who slapped has to give the top card of their own pile face down to the person who played the card that they wrongly slapped. That person adds it to the bottom of their own pile.
If you run out of cards you no longer play a card when it is your turn, obviously. You are still entitled to take part in the slapping of the next jack and if you win you are back in the game. If someone else wins, or if you meanwhile slap a card that is not a jack, you are out of the game.
When one player has all the cards, that player wins. If you wish, you can play with the time limit, and if no one collects all the cards before then, the person holding most cards when the time limit is reached is the winner.
This British game was described to me Osiris Atom-Ra under the name Slapjack, though it is more commonly known as Irish Snap.
This game can be played by any number of players - but is best for 4-8 players. The aim is to lose all your cards.
A normal pack of 52 cards is dealt clockwise face down to all players. The players are not allowed to look at their cards.
The dealer starts by placing their top card in the centre of the playing area, revealing the face of the card at the same time (as you do in snap). (See Slapjack above for suggestions on how to do this fairly.) As the card is being put down the dealer shouts 'Ace'.
Play then moves clockwise and the next player puts their card down on top of the first card and shouts 'Two'. The next player puts theirs down on the centre pile and shouts 'Three' and the next player does the same and shouts 'Four' etc... After 'Ten', 'Jack', 'Queen', 'King', the calls continue with 'Ace', 'Two' and so on.
If it so happens that the card you put down matches the card value you shout out then you and all the other players must "slap" the pile by putting one hand on the pile of cards, palm downwards. The last player to do so - the player whose hand is on top - must pick up the entire heap of played cards from the centre and add it face down to the bottom of their own pile of cards. Play begins again with the player immediately to the left of the player who picked up (play starts again with the player shouting 'Ace').
This continues until there is only one person with cards to put down. That person is the loser.
Note: if someone puts their hand on the pile and the face value and the value shouted don't match, then they must pick up the entire heap.
Trevor Redmond reports a variation of this game in which the aim is to find a winner, rather than a loser. In order to win, you have to play all your cards and then slap the pile one more time at the next legal opportunity. If, having run out of cards, you are the last to slap, or if you slap when you should not, you have to pick up the pile as usual and the game continues.