To help celebrate the fact that some teachers have been using the original Portal to teach their students valuable puzzle-solving skills, Valve is giving the game away until (and I believe including) tomorrow at Steam. If you don’t already own the game, you can simply head on over to the Steam product page and click “Install Game”.
This isn’t the first time Valve has offered Portal for free; the last time the deal was available was with the launch of the Mac Steam client. Kind of like Team Fortress 2 at the end of its paid reign, it seems Valve doesn’t mind giving away Portal at random occurences to help attract even more people to its Steam service – or, to turn people towards Portal 2.
Either way, if you have not yet played Portal, you are seriously missing out on some quality gaming. I recommend taking advantage of this deal while you still have the chance.
Thanks to marfig for the heads-up on this!