It seems that the praise I heaped upon Prey for bringing new things to the stale first-person shooter genre is already reached its expiration date, courtesy of Valve.
Valve has always beeen known for being at the forefront of the FPS genre, if not for story and action, then for cool physics and gadgets like the Gravity Gun. Enter the Portal Gun.
Rather than try and explain it, just watch the video on this page. And yes, that is a trailer for a mini-game within a game.