Werewolf is a very fun bluffing game. It has been a standard part of every tech conference I’ve been to in the past year and a half. We know people will want to play at Ignite.
It’s very similar to Mafia; it just has better branding. If you aren’t sure what I’m talking about (or don’t understand the t-shirts) here’s a brief explanation of the game:

A game of Werewolf lasts about 30 minutes and is played through alternating day and night rounds. There are two teams: the Villagers and the Werewolves. At the beginning of the game people are secretly assigned roles via the Moderator. Except for the Moderator, every person is either on the Villager’s side or the Werewolve’s side. The werewolves hide their true nature and try to secretly kill the Villagers at “night”; the Villagers attempt to root out the Werewolves during the “day”. Once a player is dead they are out for the rest of the game. The game ends when either all of the Werewolves are dead or the number of Villagers equals the number of Werewolves. The Moderator’s job is to be neutral and keep the flow of the game going.

For a more complete description of the game (including a number of variants and roles I didn’t cover) check out Zarfhome’s page.