This is another game that's on my list of fantastic party games. It's fast and fun and takes about 30 seconds to learn how to play. Essentially each player is dealt a certain number of cards. On each card is a particular commodity - ranging from oranges to wheat. Each commodity is given a certain point value. The object of the game is to corner the market on any given commodity. The way you do this is what makes Pit a great party game - you yell what you want to trade, and other players yell back whether they want to complete the trade or not. EVERY player does this all at the same time, so the result is a very loud, very hectic game where every player is engaged in play at all times.
You have to propose and make trades, all while keeping tabs on what the other players are proposing and trading. It's really really fun. You can also play with a "bear" and "bull" card that will either kill your hand or help you win.
I suggest getting the Delux version of the game as the cards are little nicer and it comes with a small bell (like the kind you find on the counter at a hotel). When a player corners a commodity, they ring the bell and the hand ends. You score points depending on which commodity you cornered. The first player to a specific score after a series of hands wins.
This game is great to play in combo with other great party games like Apples to Apples and Cranium. I don't know a single person who doesn't like to play any of these games.