TTT is essentially Mafia as a FPS, in Garry's mod.
A small group of the players are traitors, and seek to kill everyone else. They know each other, have secret communication, and some special items they can buy. The rest don't know who these are, have only public coms, and have only what weapons they can find on the map.
With the help of a detective (who can also buy some (different) special items and is known to be innocent) they use what they see, what they can find out from corpses (although the detective is better at this), and what they can work out from what people are saying and where they are to find and eliminate the traitors. However, if they choose wrongly and shoot an innocent, they do less damage in future rounds, until redeeming themselves by killing traitors.
If people like the sound of this, and we can get / have it on a server, I was thinking of holding a game one weekend evening in the near future.
EDIT: You do need BOTH Garry's mod AND CS: S installed to get it to work.
ONE MORE EDIT: Mic's will be useful. You can get away without one easily enough, but not being able to yell the name of your killer as you die is annoying, and people often get edgy and shoot if the reply isn't quickly for-coming.
# Players so far: 8