If for some reason you still haven't played the original Portal, you have no excuse. Valve is allowing you to download Portal for free onto your PC or Mac until September 20th. They are calling this initiative "Learn with Portals."
I did review Portal a while back if you are interested. It's a great game that should take you less than a day to play, so I recommend you jump on this while you have the chance.
Follow the link below to get your free download.
Hurry, only available for a couple more days!
Learn With Portals