Set

Your head will hurt when you start playing this game.  I don't mean as you play this game; rather, your head will start hurting when you first encounter this game.

From the Wikipedia article about Set:

Several games can be played with these cards, all involving the concept of a set. A set consists of three cards which satisfy all of these conditions:

• They all have the same number, or they have three different numbers.
• They all have the same symbol, or they have three different symbols.
• They all have the same shading, or they have three different shadings.
• They all have the same color, or they have three different colors.

The rules of Set are summarized by: If you can sort a group of three cards into "Two of ____ and one of _____," then it is not a set
Given any two cards from the deck, there will be one and only one other card that forms a set with them. One example of a set would be these three cards:

• One red striped diamond
• Two red solid diamonds
• Three red open diamonds

In one game, the dealer lays out cards on the table until either twelve are laid down or someone sees a set and calls "Set!" The player who called "Set" takes the cards in the set and the dealer continues to deal out cards until twelve are on the table. If a player sees a set among the twelve cards, s/he calls "Set" and takes the three cards, and the dealer lays three more cards on the table. It is possible that there is no set among the twelve cards; in this case, the dealer deals out three more cards to make fifteen dealt cards, or eighteen or more, as necessary. This process of dealing by threes and finding sets continues until the deck is exhausted and there are no more sets on the table. At this point, whoever has collected the most sets wins.

A friend introduced the game to a group at a regular family game night at a now-defunct comics and game store I used to go to, and for everyone who didn't know the game, their heads actually started to hurt.  The key to Set is to let the rules sink in -- approximately one week should does it; not even playing during that week.  The next time you play it, you'll start to get it.  It won a Mensa award, and you by all means don't have to have the Mensa-level I.Q. in order to play it.

You can pick it up at various hobby stores, online, and even at Borders.  There are also many variations and other games at The Set Company.  There's also a version for the mobile phone that you can download.