Werewolf 359 is an online implementation of werewolf/mafia/assassin/whatever. As Wikipedia sums it up,
Players are secretly assigned roles: either “mafia”, who know each other; or “townspeople”, who know only the number of mafia amongst them. During the night phase of the game, the Mafia choose an innocent to kill. During the day phase, all players debate the identities of the Mafia and vote to kill someone whom the majority suspect. Players are eliminated until either all mafia are killed or the mafia outnumber the innocents. Mafia is rarely played in groups of fewer than five, and must always start with more innocents than mafia.