Guild of Writers

The game Portal is available for free until May 24th.
http://store.steampowered.com/freeportal/

Anyone who has not played this game needs to do so immediately. You have no excuse now, it's free!

And it's a great game
Hopefully, this free version is not a lie... but wait, I got it, and it works

And to clarify--it's free as in "You download it now, you get to have it free forever" not as in, "It's free until May 24th, at which point you have to start paying for it." A good catch on Whil's part.

Please note that you don't get access to Source SDK (and by extension the level editor and various modded levels) unless you purchase some other Source game (as in pay money for it).