This is the most interesting and original game I saw at PAX so I figured I would give it its own thread. It's an indie game still in development so the graphics aren't exactly anything to talk about but the concept is brilliant.
It's an asymmetric match between two players. One player takes on the role of a spy at a party (wow, hence the name) with a number of AI-controlled guests and is given various objectives to accomplish. The objectives include things like planting a bug on an ambassador attending the party, contacting a double agent by saying a key phrase, etc. The other player is a sniper who is supposed to kill the spy. But he doesn't know who the spy is. So the player controlling the sniper is trying to carefully observe the guests at the party to look for tells that give away the spy (for instance some of the spy actions have distinctive animatiosn) while the spy is trying to accomplish his goals while being as subtle as possible. If you're a spy you don't want to rush straight for your objective but rather stop and mingle with the party guests and basically try to act like the AI. There is some really deep, subtle gameplay here. Just as an example, holding something (like a drink) can prevent the spy from carrying out certain actions, so it's generally undesireable. On the other hand, since it's undesireable you might want to do it deliberately to reduce suspicion. But without being too obvious about it...
Unfortunately there is no public demo available at this point. They are doing sign-ups for a limited paid beta (following the Minecraft model) but they're still keeping the number of beta testers small at this point.