Portal 2. Sigh. So mainstream.
Try the authentic 2D experience at online gaming site Kongregate. It’s a Flash version. You’ve probably never heard of it.
Portal: The Flash Version includes over 40 challenging portal levels, with almost every feature the real game does, in 2D – energy balls, cubes, turrets and even the famous crusher from the trailer. The game also includes a console to mess around with after finishing the game, or just being frustrated thinking with portals.
(Thanks Mike Casey)