Hmm. Right now it says the price of Portal is Free. Can anyone that doesn’t own it confirm that it is free?
They did do a free giveaway of the game a while back. Maybe it’s on again? Or soon will be?
Edit: If you have Steam open try clicking on this link. Or go to the store page in your webbrowser and select the Install Game button. Don’t do it from the Steam Store Browser.
UPDATE: Looks like it is indeed FREE according to this link: http://www.learnwithportals.com/