Mafia

A game of good vs. evil, Mafia is a game of deception, perception, and murder. The setting is of a small town that has discovered that there is a secret mob amongst their citizenry and must hunt them down. The players are assigned one of two roles: Townspersons (good) or Mafia (bad). However, each player is only sure of their own role, making the mafia's identity a secret.

During the day all players must try to determine which player among them are secretly part of the mob. Then they vote to lynch one player. That player is killed (removed from the game) and their role is revealed. With any luck, it's a mafia member; otherwise, an innocent townsperson is killed. Then night phase begins and the members of the mob secretly communicate to decide which player to kill. All the players "wake up" the following day to discover the dead body and start the voting process over. Victory conditions are simple: be the last group standing. It is a game of the uninformed majority vs. the informed minority.

Now, this game has become very popular in forum play, its main home being here. When played on-line the game is strictly about post analysis and vote tracking. Also, voting in the online game is done at the townspersons' discretion typically, move their votes when they choose until a majority of the votes are on one player. That person is instantly lynched in game.

For more about strategy, you can check out the wiki. It's not organized very well, nor complete, but has many of the basic concepts, along with a listing of all special roles that are used to add extra twists to the game.