Anyone tried
SpyParty?
While it has the drawbacks of being in beta and not on steam (yet) it looks like a lot of fun already so pondering picking it up.
Basically it is a two player game where one player is a spy at a party among other NPCs, trying to accomplish missions, while the other player has to figure out who the PC among all the NPCs is, and deliver some swift justice in the form of a sniper rifle bullet.