If you have heard of Blender then you probably know that it is a free opensource 3D modelling program that also has a game engine.
Dead Cyborg is probably the most graphically beautiful game I have seen with the Blender Game engine.
Episode 1 & 2 are totally FREE and there is a Linux release!
Dead Cyborg is a first person sci-fi puzzle game. No weapons, just explore and solve puzzles.