After seeing Mist's Crazy Eights game, I thought I'd post the rules for a similar game I invented last week on the ferry to Wales. It's called Agility, and here are the rules:
Start by dealing each player (you need at least two, but you won't be able to accommodate more than 5) a hand of 10 cards. The cards must be dealt face down, and player's aren't allowed look at them! Take 1 more card from the deck and place it face down. This is the Pile. When everyone is ready, the game begins with the dealer flipping the Pile card face up, at which point players may look at their hands.
Players can place a card from their hand on top of the Pile if their card has either the same number or suit of the topmost Pile card, and when a player runs out of cards in their hand they win. If a player can't play any cards from their hand, they may draw a card from the deck. When the last card in the deck is drawn, whoever has the least amount of cards in their hand wins.
So, why's it called Agility? Because this isn't a turn based game! Whoever's fastest at discarding cards and building chains of their suits and numbers will emerge victorious. When you get 2 veteran players facing off, matches usually last around 20-30 seconds!