It appears that as of a few hours ago, Valve dropped the (already low) price tag on Portal and has made the game free to play on Mac and Windows.
The standalone version of portal is now showing up with no price, and just an "install" button. It's not clear whether or not Valve is planning on going down the path that it took with Team Fortress 2 earlier this year, making the game free to play and using micro-transactions to make money out of it on an ongoing basis.
If you've still not played Portal, and have been living under a rock, the game description says;
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 one of the most popular valve games of all time, and is based on the Half Life 2 engine. To get the game now, for free on Mac or PC, click here.