Here, I'm practically gift-wrapping this for you guys.
THING
We played it a couple times in chat and it's very fun. Some modifications need to be made for the forum though, so I'll outline them now:
1. Of the group you begin with, only two of you are a shape-shifting monsters. The monsters infect a person at night and replace him or her with a doppelganger who fights for them.
2. The days are devoted to voting on someone to be scanned. If a normal is scanned, nothing happens. If an infected person is scanned, that person is quarantined and is not allowed to vote on future days. If a monster is scanned, the same thing happens as if he or she were infected, with the difference being that the monster can no longer convert in night phases.
3. Since each monster gets to infect one person a-piece the first night, they start with four people on their side. This means you have to have a decent sized group. It also prevents the game being over in one turn in case someone makes a lucky guess and picks the monster right off.
4. Realistically the monsters only have to outnumber the mortals in order to win, as they can affect the voting to win every time. For this reason, a GM may want to power up certain players (for example, one player gets a secret extra vote, or a player can scan at night as well but has no power to quarantine on his or her own) among the mortals to even the odds. It also depends on how big a crowd signs up for the game.
5. This is the most important point. Without dredging things up to make anybody feel bad, I'm simply saying matter-of-factly that if you recruit a player to be one of the monsters, MAKE SURE THEY WILL ACTUALLY PLAY THE GAME. Without that the game becomes sort of pointless.
I would run the game myself but I am probably going to be gone again after tomorrow's recording of the podcast. So, someone should definitely take up the reins for this game and give it a whirl, if it will be a while before Mafia happens again or if people are sort of burned out on that game.