If you are one of the last two people left in this world who still hasn't bought Portal, then what the FUCK are you waiting for?! Oh, you were waiting for it to be free. You could have been waiting a pretty long time, but today is your lucky day because it is free on Steam! You will however have to act fairly quick. It's only free until September 20. If you are to lazy to find it yourself, you can just click here and download away.
source:Learn With Portal