Werewolf (also known as Mafia) is a game of guess-who for four or more people; groups of ten to twenty seem to work best. Requires a person willing to be the narrator, and traditionally a deck of cards is used to randomize roles, though you can do this in many ways. Highly customizable!

Yotam Haimberg and I ran a large play-by-email game of this in 2011: https://etcmafia.wordpress.com/about/ and blog post