Developer: Chris Hecker (Twitter)
Platforms: Windows, Mac
DRM & Distribution: Game account activation
Development Status: Beta
You can find a good description of this game on its Wikipedia Page. Basically, one player, the spy, has to try to complete a number of set interactions in the time limit of the game without alerting the other player, the sniper, to the fact that he is the spy. It sounds pretty neat, and after watching the video below of two guys playing it, I have to say it looks pretty entertaining as well.
In SpyParty, the player takes control of either a sniper or a spy in a two player match. It is the spy's objective to complete a certain amount of missions within a given time limit while the sniper has the objective to identify and kill the spy with his one shot before the missions are completed. When a match is created between two players the player who has selected to be the spy has the option of choosing the map and the mission objectives for the game, with potential handicaps for either the spy or sniper depending on how many objectives are shown or need to be completed. After both players have agreed upon the game, the match is launched.
If you can't see the video below in Chrome, click the shield beside the URL and click 'load unsafe scripts'