Mafia is a forum game in which there are two sides, the town and the mafia. The mafia are the minority, they know their fellow members, and they kill town players to try and take control. Meanwhile the town have to decide who is a mafia member and vote to get rid of them. The object of the game is, for the most part, to lie your pants off.
A wiki on the game can be found here and a short, condensed version can be played at epicmafia.
I'm going to start a game for CWers in about a week once I get back from holidays. If you want to play please email email@example.com to say so. You'll need to be able to post in this thread fairly regularly and communicate once every couple of days by email.