Not the secretive family-type thing, but the party game. It's a game I picked up a few years back when I was with YFC. It's pretty much the best clean fun you can have with a mixed group of 6-20 people. It is simply the business.
And it's so clever I couldn't remember how to play it, and had to reach for my trusty Google. "Easy," thinks me. "I'll chase down a copy of the rules, and we can play it with the teenagers tonight." (This is Friday past.) Try it. Hit Google and search on something like 'mafia group game'. Loads of hits, as it turns out that there's somewhere in the region of nine hundred thousand variations of this game. Not only that, but it appears to be at the root of this completely unexpected subculture!
And there's a web page devoted to each variation, but I still intend to put up an outline of the way we played it on Friday, because it was fun and it's about time I put something useful on this site.