It's a mod of Portal 2, free on Steam with a 9/10 rating (2300+ votes) and awesome trailer:http://store.steampowered.com/app/317400/
I've just started playing and it's been pretty amazing so far. The puzzles are really well-crafted and harder than those from Portal 1 and 2, which makes the satisfaction of solving them so much greater. Gameplay is definitely on par. Everything - audio and visuals - is extremely professionally done. Its only weakness is that its humour never quite rises to the surreal genius of the original Portal games, but that would have been asking too much.