http://store.steampowered.com/freeportal/ No longer free.
Portal used to cost money. Until May 24th, it's free. End of story.
Well, technically speaking, there are some strings attached. Fortunately, they're entirely decorative. We just like the way they look, swaying in the gentle breeze created by a million people simultanously fainting from shock at the news that Portal is free. Now you have no reason not to try Portal.
"I have a reason," some of you are probably typing into an angry email. "You see, sir, I own a Mac."
Well guess what: For the first time ever, Portal is also available for the Mac.
Portal is a puzzle / action game set in the Half Life universe, the story takes place in a secret research facility owned by the fictional company Aperture Science, Inc. The game follows an unnamed character (and only visible by creating "portal mirrors") as she is lead through a series of dangerous tests by a sinister robotic voice known as GLaDOS. The character must use inter-dimensional portal technology to overcome the challenges of the test chambers and come closer to discovering the mysteries behind GLaDOS and Aperture Science, Inc.
So, if there is any poor soul that has not played this yet, this is your chance!