Steam now indicates that their smash hit Portal is now available for free on Steam. If you haven't got it, now is your chance. In case you have been living in a country without Internet or electricity:

Portal™ is a new single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.

The game is designed to change the way players approach, manipulate, and surmise the possibilities in a given environment; similar to how Half-Life® 2's Gravity Gun innovated new ways to leverage an object in any given situation.

Our game page has more, including screenshots and trailers.

Update: I've just tried to add this game to my account, but it doesn't seem to work - you can click 'install', but you just get the "working" icon in the corner and it doesn't prompt me to install the game as usual. Anyone else having any success?

Update #2: The price is now back to USD$20, so looks like it was just a glitch in their pricing system. Doh!