Well, since Portal is a FPS, this should be the correct section. For those of you who can't play the original game for one reason or another, you can play a 2D version of it for free, from Newgrounds. This game should keep you busy for at least an hour. I got around to level 30 or so and decided to quit.
Here's a direct link: http://uploads.ungrounded.net/404000/404612_Portal.swf