A version of Quake being developed by a research team in Wearable Computing Lab of the University of South Australia.
AR in ARQuake stands for augmented reality.
Stated simply, the idea of this Quake version is that the "levels" of the game are normal, everyday environments, and players move through the area normally wearing AR gear - and then, there's monsters literally around every corner. The game engine has data on the structures on the area, and has sensors to detect the player's movement using digital compasses, inclinometers, GPS and pattern recognition. The player has transparent computer display glasses on which the picture is projected.
Apparently the system is showing some progress; Actual screenshots were somewhat rough, but clearly recognizable. As of yet, the game playing may be somewhat hindered by the huge amount of computer and tracking equipment carried with the player, but the gear seemed surprisingly well packed...
Home page of the project: http://wearables.unisa.edu.au/arquake/