This game seems to have arrived with much less fanfare than it deserved.
For those of you who don't know about it, this was the game released alongside episode 2 and team fortress 2 that valve included in their orange box. Portal focused on a "gun" that let the user create an interdimensional portal on most flat surfaces. The user had two portals (blue and orange) to "fire" at the walls, and each portal let to the other. This introduced quite a new twist into FPS gameplay. Sure it was done in previous titles like Prey, but for the first time, the user is able to control and manipulate them in ways even the creators cannot imagine.
I think it would be interesting to discuss the ways that gameplay could integrate such a marvelous feature. I feel like portal itself didn't even fully explore what they could have done with existing resources. I was half expecting to
take out some zombies, or even some combine soldiers! But not as much as a headcrab
more info on portal at
http://en.wikipedia.org/wiki/Portal_%28video_game%29