Portal is free until Sept. 20

Good day, test subjects. Valve is currently offering its first physics-fracturing puzzler, Portal, for free until September 20. Valve was apparently inspired by the way educators are using Portal to teach mathematics, and is giving the game away to engage more students in the sciences ... and cake.

So, if for some reason you've missed out on one of the most praised games of this generation, here's your chance to pick it up on PC or Mac for free (you monsters).

[Thanks, Rahul.]