Werewolf is a simple game for a large group of people (seven or more.) Two of the players are secretly werewolves. They are trying to slaughter everyone in the village. Everyone else is an innocent human villager, but one of the villagers is a seer (can detect lycanthropy). Some people call it a party game, but it's a game of accusations, lying, bluffing, second-guessing, assassination, and mob hysteria. Sounds like a blast to me. [via metafilter]
I recently participated in a similar game called "The Mole" in which there are two teams which are trying to complete certain tasks, except that there's a sabateur (a "mole") on each team. Of course, my team emerged victorious, thanks mostly to a brilliant strategy in the opening round, resulting in a commanding lead for my team. The other team became a little bitter about that, as evidenced by this highly biased, but also hilarious mock review of the event (I am the one referred to as "Mark" in said review).